Horus

Horus, otherwise known as Horus Lupercal, or the Lupercal, and ultimately, the Warmaster, was one of the 20 genetically-engineered Space Marine Primarchs created by the Emperor of Mankind from the foundation of his own DNA before the start of the Great Crusade to lead the armies of the newborn Imperium of Man. Horus was the Primarch of the Luna Wolves Legion of Space Marines (later renamed the Sons of Horus), the first Imperial Warmaster, the most favoured son of the Emperor of Mankind, and ultimately, the greatest Traitor in the history of Mankind. Horus' homeworld was the Hive World of Cthonia which lay only a few light years from Terra. Horus was thus the first Primarch to be rediscovered by the Emperor after the Great Crusade began in the early 31st Millennium. Horus was responsible for unleashing the horrific seven year-long civil war known as the Horus Heresy upon the Imperium of Man in the early 31st Millennium which killed billions of men, women and children in pursuit of his mad ambition to overthrow the Emperor of Mankind and replace him as the ruler of the human race. While Horus ultimately lost his bid for power and was slain by the father he had once so loved during the Battle of Terra, his actions damaged the Imperium of Man beyond repair. What followed is the current Age of the Imperium, where Mankind is beset by countless horrific dangers to its existence within and without, and the Imperium itself has become a stagnant, repressive, intolerant and dehumanising galactic presence.

Early Life
Created as a genetically-engineered organism by the Emperor in the Imperial gene-laboratories under the Himalayan Mountains on Terra in the late 30th Millennium, Horus, along with his brother Primarchs, were scattered across the Milky Way Galaxy through the Warp by the machinations of the Ruinous Powers of Chaos. This is said to be when the Dark Gods first planted the seeds of heresy into the infant Primarch, whispering darkly into his soul and tempting him to their cause. The capsule carrying the infant Horus came to rest on the Mining World of Cthonia, the primary planet of a star system within a reasonable slower-than-light distance of Terra. Whereas the early history of many Primarchs is extensively if unevenly documented, the same cannot be said of Horus. Contradiction and omission tarnishes all accounts of Horus' formative years. It is clear that the Emperor did find Horus and also that he took command of the XVI Legion early in the Great Crusade. Beyond these manifest facts, agreement between the early Imperial sources is decidedly lacking, some even placing Horus on Cthonia as a foundling.

Like many of his superhuman brethren, these sources say that the young Primarch thrived in Cthonia's harsh environment, learning his first lessons in war and killing from Cthonia's tech-barbarian kill-gangs. The world of Cthonia had been settled in the very earliest days of Mankind’s exploration of the stars, its rich natural resources ruthlessly exploited until they were all but played out. Thus, Horus grew to maturity amongst the anarchic gangers that populated the post-industrial nightmare of a world honeycombed with long-extinct mines and dominated by decaying hive cities. Though Horus had not been raised during his formative years on Cthonia -- uncommonly, for a Primarch, he had not matured on the cradle-world of his Legion -- he spoke the harsh language known as Cthonic fluently. In fact, he spoke it with the particular hard palatal edge and rough vowels of a Western Hemispheric ganger, the commonest and roughest of Cthonia’s feral castes. Later on, it always amused some of the Battle-Brothers within the XVI Legion to hear this accent. They assumed that Horus spoke in this manner because that was how the Warmaster had learned the language, from just such a speaker, but many would come to doubt this hypothesis later on. Horus never did anything by accident, and there were those who believed that the Warmaster’s rough Cthonic accent was a deliberate affectation so that he would seem, to the Astartes of the XVI Legion, as honest and low-born as any of them. It was from the hyper violent gang-scum of Cthonia that many of the earliest inductees into the Space Marine Legions were recruited, and it was there that the Emperor had found the first of his lost sons.

Another source claims that Horus returned to Terra itself. It is said that Horus grew at the Emperor's side, learning from his father even as they took back the Sol System and forged the alliances between the techno-barbarian nations of Terra and with the Mechanicus of Mars that created the Imperium of Man. Other highly creditable claims state that the Emperor found Horus, the first of his lost sons, but neither source specifies where, or the location of this finding. Surrounded in millennia of myths and allegory, the truth of Horus' origins will more than likely never be known. As a result, Horus was for many standard years the Emperor's only son, and there was a great affinity between them. The Emperor spent much time with his protege, teaching and encouraging him. Horus was soon placed in command of the XVI Legion, which had already come to be known as the Luna Wolves -- 10,000 Astartes created from his own genetic code. With these superhuman warriors to lead, Horus accompanied the Emperor for the first thirty standard years of the Great Crusade that had begun in ca. 800.M30, and together they forged the initial interstellar expansion of the young Imperium of Man.

Great Crusade
Cthonia, relatively close to Terra in the void, and with whom some minor intermittent contact had been maintained even through the Age of Strife, had its murderous and strife-torn population marked by one of the first expeditionary fleets to leave the Sol System. With its resources stripped it had little strategic worth and its people were judged largely beyond illumination, but the fledgling Imperium needed teeth as well as ideals and that necessity saved Cthonia. Its youth was harvested in their tens of thousands, first as impressed troopers for the Crusade armies and then the finest specimens were taken for the Legion. On Luna these chosen sons of Cthonia were reborn as warriors of the XVIth Legion. With these new recruits came a further accolade. The Emperor in honour of their new birthplace, the ruthless propensities of the Cthonians and the victories of the past, gave the XVIth Legion a name to strike fear into his enemies. As they scattered to spill blood amongst unclaimed stars their enemies would know them as the Luna Wolves.

For thirty standard years, the Emperor and Horus fought the opening campaigns of the Great Crusade side by side, the Primarch learning at the foot of his sire. When at length the Emperor detected another of the Primarchs and departed to locate him, Horus was left at the head of his master’s hosts, entrusted with the command of the conquering armies. Horus was well suited to the task, and the lessons he had learned in the previous three decades served him well. As one by one the Primarchs were united with their gene-father and their brothers, Horus came increasingly to be regarded as the greatest of their number, the first amongst equals. While happy that he would soon meet one of his brothers, Horus swore to himself that he would always remain the Emperor's favoured child. The Luna Wolves waged war for two hundred years, pushing back the darkness with fire and blood. Their victories were manifold and Horus' generalship was legend, and so it was that the respect of their brother Legions rose to almost unrivalled heights.

While many of his brothers and the Space Marine Legions created in their genetic image were gifted in specific fields of military science, Horus was a natural leader, his greatest genius his ability to meld seemingly divergent allies into a coherent whole. This skill was not only of use on the battlefield, for it carried over into contacts with the peoples the Great Crusade met. It was Horus’ way to treat with the populations of newly-contacted worlds according to the cultural traditions of each, and this highly successful doctrine was repeated in each of the Imperial Expeditionary Fleets. Horus believed that to do so would lessen the hostile reaction of opponents who wished to parley. Tragically, it might also have been the cause of the Primarch’s fall, and with him fully half of the Space Marine Legions.

As the Great Crusade pushed outward, and more Primarchs were discovered, the Emperor's time became divided, pulled in more and more directions. Horus was often placed in overall strategic command of the Crusade, a position in which he proved his skill as a leader time and time again. He quickly won the approval and support of the other Space Marine Legions, along with their leaders. One of the skills that made Horus such a great leader was that he possessed an innate understanding of human psychology; he was able to read people in such a way that he could choose to promote their strengths or exploit their weaknesses. This allowed him to find a non-military solution in several campaigns as he used his sheer charisma and negotiation skills combined with the ever-present threat of the Space Marine Legions' unstoppable force to bring other human-settled worlds into Imperial Compliance without bloodshed.

His understanding of the human mind allowed Horus to bring out the best within his fellow Primarchs, which allowed him to deploy the various Legions into the battlefield roles to which they were best suited. He quickly learned of the skill displayed by the White Scars and Night Lords in deploying for rapid strikes, while the Imperial Fists and Iron Warriors were always placed at the forefront of planetary sieges. Horus was said to wield the Space Marine Legions, and later the human soldiers of the Imperial Army, as a lesser general would position individual squads to perform to their advantages. He was also responsible for promoting competitive rivalries between certain Space Marine Legions in the desire to spur these Astartes on to greater heights, but these rivalries would eventually transform into outright hatred when the Great Crusade took a turn for the worse.

Triumph of Ullanor
"You are like a son and together we have all but conquered the galaxy. Now the time has come for me to retire to Terra. My work as a soldier is done and now passes to you for I have great tasks to perform in my earthly sanctum. I name you Warmaster and from this day forth all of my armies and generals shall take orders from you as if the words came from mine own mouth. But words of caution I have for you, for your brother Primarchs are strong of will, of thought and of action. Do not seek to change them, but use their particular strengths well. You have much work to do, for there are still many words to liberate, many peoples to rescue. My trust is with you. Hail Horus! Hail the Warmaster!"

- The Emperor of Mankind, during the Triumph of Ullanor

Though the Luna Wolves had won many victories in their years of ceaseless conflict, one would eclipse all others and see them reborn once again. The greatest of the nascent Imperium's victories during the high point of the Great Crusade came in the form of the defeat of the largest Ork empire ever encountered. The Ullanor Crusade was a vast Imperial assault on the Ork empire of the Overlord Urrlak Urruk. The capital world of this Greenskin stellar empire, and the site of the final assault by the Space Marine Legions, lay in the central Ullanor System of the galaxy's Ullanor Sector. The Crusade included the deployment of 100,000 Space Marines, 8,000,000 Imperial Army troops, and thousands of Imperial starships and their support personnel. The Ullanor Crusade marked the high point of the Great Crusade's vast effort to reunite the scattered colony worlds of humanity. The Orks of Ullanor represented the largest concentration of Greenskins ever defeated by the military forces of the Imperium of Man before the Third War for Armageddon began during the late 41st Millennium. Following the defeat of the Orks of Ullanor, the Emperor of Mankind returned to Terra to begin work on his vast project to open up the Eldar Webway for Mankind's use. In his place to command the vast forces of the Great Crusade he left Horus. In the aftermath of this Ullanor Crusade, Horus was granted the newly-created title of "Warmaster," the commander-in-chief of all the Emperor’s armies who possessed command authority over all of the other Primarchs and every Expeditionary Fleet of the Great Crusade. Before returning to Terra to oversee the next phase of the creation of his stellar empire, the Emperor suggested to Horus that he rename the XVI Legion the "Sons of Horus," in honour of their Primarch and to show his preeminent place amongst the other Primarchs. Horus initially declined this honour, not wishing to be set above his brothers, and so his Legion continued as the Luna Wolves for a little while longer. But Horus and the other Primarchs never came to terms with the Emperor's absence. Their hurt feelings over his seeming abandonment of the Great Crusade to pursue a secret project whose purpose he chose not to reveal to his sons laid the seeds of jealousy and resentment that would ultimately blossom into the corruption that begat the Horus Heresy.

Corruption by Chaos
Shortly after the Luna Wolves Legion and their allies declared victory in the great Ullanor Crusade against the mightiest Ork empire faced by the Imperium until the 41st Millennium, the Emperor offered Horus the honour of renaming his Legion the Sons of Horus in honour of himself and to show his preeminent place among the other Primarchs. Horus refused this honour, characteristically not wishing to be set above his brothers, but was promoted to the newly-created rank of Imperial Warmaster, serving as the new Supreme Commander of the Imperium's millions-strong armies while the Emperor left the Crusade in Horus' capable hands and returned to Terra to pursue the secret Imperial Webway Project that he hoped would draw the newborn Imperium together in unbreakable bonds.

Despite the majestic rank and unparalleled authority bestowed on him, it was said that Horus was not content. The wording of the Emperor's declaration, claiming the glory of Horus' victories as entirely his own, chafed. Although this was the usual rhetoric for such Imperial announcements, Horus saw that while the Emperor remained behind in the Imperial Palace on Terra for reasons that he would not share even with his sons, Horus would be out in the field of battle, winning the Emperor's Imperium for him. It seemed that a deep-rooted resentment and jealousy had begun to simmer in the corners of the Warmaster's mind. As the Imperial Warmaster, Horus took over command of the Great Crusade, and accepted his new duties with earnest dedication. However, there was much dissension in the ranks of the Primarchs and other parties in the Imperium over the Emperor's decision to withdraw from the campaign and return to Terra as well as to reorganise the political administration of the Imperium under the control of a Council of Terra headed by his regent, Malcador the Sigillite. Only a handful of the Primarchs, amongst them a scheming Lorgar, remained steadfast beside the Warmaster during this period of conflict. Horus also disagreed with many of the decrees passed by the newly established Council of Terra, a ruling body of Imperial nobles and bureaucrats, which were intended to shift the burden of taxation and administration onto the newly-conquered Imperial Compliant worlds. Even worse, Horus came to believe in his heart that he was failing his father, and was deeply wounded that the Emperor had revealed to none of the Primarchs, not even his most favoured son, why he had secluded himself upon Terra and the truth behind his secret Imperial Webway Project. These seeds of bitterness, resentment and frustration grew, and would soon bear deadly fruit.

First Chaplain Erebus of the Word Bearers, the XVII Space Marine Legion that was secretly in league with the Forces of Chaos since they had been humiliated by the Emperor for their violations of the Imperial Truth on the world of Khur over 40 years before the start of the Horus Heresy, became a very close confidante and adviser of Horus. The Words Bearers, who originally had worshipped the Emperor as a God and whose Primarch Lorgar had penned the Lectitio Divinitatus before he turned against his father following the events on Khur, had been brutally reprimanded for their constant erection of temples and shrines dedicated to the worship of the God-Emperor on newly conquered worlds, a policy that had directly violated the Emperor's secularist philosophy and had slowed the XVII Legion's progress in the Great Crusade to a crawl. In the wake of his humiliation and the reprimand against his Legion on Khur, Lorgar had sought to discover whether there truly were divinities worthy of human worship during the quest that came to be known as the Pilgrimage of Lorgar. During this journey, Lorgar entered the Eye of Terror and came face-to-face with the power of Chaos, a force that he believed was truly divine and worthy of his service and the worship of all Mankind. He brought this faith to his Legion, and soon all the Astartes of the Word Bearers believed that the Chaos Godswere more worthy of their loyalty and worship than the Emperor who had proved himself to be a false god. With the guidance of the Ruinous Powers, Lorgar and the Word Bearers hatched their secret plans over the course of the next four decades to convert humanity to the service of the Dark Gods, starting with Horus. After Lorgar placed his agent near Horus, Erebus slowly managed to twist the thinking of Horus against the Emperor and turn half of the Mournival, the Warmaster's most trusted advisers within the XVI Legion, towards the same path of treachery by exploiting their intense loyalty to their Primarch over that for their far more distant Emperor. This plot reached its climax on the moon of the Feral World of Davin, where Horus was wounded during a struggle against Eugen Temba, the former Planetary Governor of Davin who had rebelled against the Imperium following his corruption by Chaos. An ancient xenos artefact blade sacred to Nurgle known as the Kinebrach Anathame had been stolen by Erebus from the museum of an advanced human civilisation called the Interex during the Luna Wolves' brief sojourn on their world of Xenobia and had been given by him to Temba. Temba had become the mutated servant of Nurgle, the Chaos God of disease and decay, and managed to seriously wound the Warmaster with the corrupted blade. The Apothecaries of the XVI Legion proved incapable of healing the Warmaster's wounds despite every form of advanced medical technology at their disposal, until it seemed that Horus would surely die.

Erebus then convinced the Luna Wolves' warrior lodge to allow him to take the Warmaster to a secret sect on Davin and use sorcery at the Temple of the Serpent Lodge in order to treat the Warmaster. The creation of a warrior lodge within the Luna Wolves and many other Space Marine Legions based on the lodges used by the savage warriors of Davin had been another of Erebus' ploys to infiltrate and corrupt the Astartes into turning against the Emperor. However, not all Astartes of the Legions joined the lodges, as many saw them as a direct violation of the Emperor's desires that all Space Marines dedicate themselves to truth and openness. The lodges would prove to be the primary purveyors of corruption in those Legions which turned Traitor in the days immediately preceding the start of the Horus Heresy and those Astartes within those Legions which lodges which held themselves aloof from those secretive organisations were marked as Imperial Loyalists who would later be betrayed at Istvaan III.

The secret sect on Davin was in truth a Chaos Cult, and using sorcery, which was outlawed by the Emperor, the cultists managed to warp the mind of the Warmaster against the Emperor by playing on the seed of jealousy and resentment that he felt for his father after the Emperor had left the Great Crusade behind to return to Terra. During the dark rituals that followed within the temple, Horus' spirit was transferred from his body into the Immaterium. There, he bore witness to a nightmare vision of the future. He saw the Imperium of Man as a repressive, violent theocracy, where the Emperor and several of his Primarchs (but not Horus) were worshipped as Gods by the masses. While this vision of the Imperial future granted by the Chaos Gods was a true one, it was ironically an outcome largely created by the Warmaster's own actions. The Dark Gods portrayed themselves as victims of the Emperor's psychic might, and claimed that they had no real interest in the happenings of the material world. Magnus the Red, the sorcerous Primarch of the Thousand Sons Legion, had also travelled into the Warp via sorcery to try and stop Horus from turning to Chaos. Magnus explained that the Warmaster's vision was only one among many possible futures, but one that Horus alone could prevent. Horus, already jealous and resentful of the Emperor, proved all too receptive to the Ruinous Powers' false vision. The Chaos Gods' pact with Horus was simple: "Give us the Emperor and we will give you the galaxy." Driven by his jealousy, desire for power and anger at what he saw as his father's abandonment of him, Horus accepted the Ruinous Powers' offer. They healed his grievous wound and filled him with the powers of the Warp. Renouncing his oath to the Emperor, Horus led his Legion, renamed the Sons of Horus, into worship of the myriad Chaos Gods in the form of Chaos Undivided. He then sought to turn many of his fellow Primarchs to the service of Chaos, and succeeded with Angron of the World Eaters, Fulgrim of the Emperor's Children and Mortarion of the Death Guard, who were the first of many to follow, along with many regiments of the Imperial Army and several Titan Legions of the Adeptus Mechanicus.

Magnus the Red, the Primarch of the Thousand Sons Legion, foresaw Horus' actions through his Legion's own use of forbidden psychic sorcery. Magnus then attempted to forewarn the Emperor of the impending betrayal of his favourite son. However, knowing that he would have to find a means of quickly warning the Emperor, Magnus used sorcery to send his message to the Emperor. The message penetrated the potent psychic defences of the Imperial Palace on Terra, shattering all the psychic wards the Emperor had placed on the Palace -- including those within his secret project in the Imperial Dungeons, where he was proceeding with the creation of the human extension into the Webway. Refusing to believe that Horus, his most beloved and trusted son, would actually betray him, the Emperor instead mistakenly perceived the traitor to the Imperium to be Magnus and his Thousand Sons, who had long suffered from a near-debilitating run of mutations because of the instability of Magnus' own genome and were known to have practiced the sorcery that had been expressly outlawed in the Imperium. The Emperor ordered the Primarch Leman Russ, Magnus' greatest rival, to mobilise his Space Wolves Legion and the witch hunters known as the Sisters of Silence and take Magnus into custody to be returned to Terra to stand trial for violating the Council of Nikaea's prohibitions against the use of psychic powers within the Imperium. While en route to the Thousand Sons Legion's homeworld of Prospero, Horus convinced Russ, who had always been repelled by Magnus' reliance on psychic powers, to launch a full assault on Prospero instead, even though Magnus had been entirely willing to face the Emperor's judgment once he realised he was being manipulated by the entities that called the Immaterium home.

Istvaan III Atroc​ity
Unknown to the Emperor, the Word Bearers Legion had been devoted to Chaos Undivided for some time before this event. The Imperial Planetary Governor of Istvaan III, Vardus Praal, had been corrupted by the Chaos God Slaanesh whose cultists had long been active on the world. Praal had declared his independence from the Imperium, and practiced forbidden sorcery, so the Council of Terra charged Horus with the retaking of that world, primarily its capital, the Choral City. This order merely furthered Horus' plans to overthrow the Emperor. Although the four Legions under his direct command -- the Sons of Horus, the World Eaters, the Death Guard and the Emperor's Children -- had already turned Traitor and now pledged themselves to Chaos, there were still some Loyalist elements within each of these Legions that approximated one-third of each force; many of these warriors were Terran-born Space Marines who had been directly recruited into the Astartes Legions by the Emperor himself before being reunited with their Primarchs during the Great Crusade. Horus, under the guise of putting down the religious rebellion against Imperial Compliance on the world of Istvaan III, amassed his troops in the Istvaan System.

Horus had a plan by which he would destroy all the remaining Loyalist elements of the Legions under his command, a plan that would ultimately unfold into the nightmare of what Imperial scholars would later name the Istvaan III Atrocity. After a lengthy bombardment of Istvaan III, Horus dispatched all of the known Loyalist Astartes down to the planet, under the pretense of bringing it back into the Imperium. At the moment of victory and the capture of the Choral City, the planetary capital of Istvaan III, these Astartes were betrayed when a cascade of terrible virus-bombs fell onto the world, launched by the Warmaster's orbiting fleet. Captain Saul Tarvitz of the Emperor's Children, however, was aboard his Legion's flagship Andronius and discovered the plot to wipe out the Loyalist Astartes of the Traitor Legions. He was able, with help from Battle-Captain Nathaniel Garro of the Death Guard who was in command of the Death Guard frigate Eisenstein, to reach the surface of Istvaan III despite pursuit and warn the Loyalist Space Marines he could find of all four Legions of their impending doom. Those that heard or passed on Tarvitz's warning took shelter before the virus-bombs struck. The civilian population of Istvaan III received no such protection: eight billion people died almost at once as the lethal flesh-dissolving virus called the Life-Eater carried by the bombs infected every living thing on the planet. The psychic shock of so many deaths at one time shrieked through the Warp, briefly obscuring even the Astronomican. The Primarch of the World Eaters, Angron, realising that the virus-bombs had not been fully effective at eliminating all the Loyalists, flew into a rage and hurled himself at the planet with 50 companies of Traitor Marines. Discarding tactics and strategy, the World Eaters Legion's Traitors worked themselves into a frenzy of mindless butchery. Horus was furious with Angron for delaying his plans, but the Warmaster sought to turn the delay into a victory and was obliged to reinforce Angron with troops from the Sons of Horus, the Death Guard, and the Emperor's Children. Fortunately, a contingent of Loyalists led by Battle-Captain Garro escaped Istvaan III aboard the damaged Imperial frigate Eisenstein and fled to Terra to warn the Emperor that Horus had turned Traitor.

On Istvaan III, the remaining Loyalists, under the command of Captains Tarvitz, Garviel Loken and Tarik Torgaddon, another Loyalist member of the Sons of Horus, fought bravely against their own traitorous brethren. Yet, despite some early successes that delayed Horus' plans for three full months while the battle on Istvaan III played out, their cause was ultimately doomed by their lack of air support and Titan firepower. During the battle the Sons of Horus Captains Ezekyle Abaddon and Horus Aximand were sent to confront their former Mournival brothers, Loken and Torgaddon. Horus Aximand beheaded Torgaddon, but Abaddon failed to kill Loken when the building they were in collapsed. Loken survived and witnessed the final orbital bombardment of Istvaan III that ended the Loyalists' desperate defence. To prove his worth and loyalty to Lord Commander Eidolon of the Emperor's Children -- and thus to his Primarch, Fulgrim -- Captain Lucius of the 13th Company of the Emperor's Children, the future Champion of Slaanesh known as Lucius the Eternal, turned against the Loyalists that he had fought beside because of his prior friendship with Saul Tarvitz. Lucius slew many of them personally, an act for which he was then accepted back into the Emperor's Children on the side of the Traitors. In the end, the Loyalists retreated to their last bastion of defense, only a few hundred of their number remaining. Finally, tired of the conflict, Horus ordered his men to withdraw, and then had the remains of the Choral City bombarded into dust for a final time from orbit.

Preparations and Allegiances
Much of Horus' later success arose from the thorough groundwork he had laid before the opening shots of the Heresy were fired at Istvaan III. He had already swayed the Primarchs Angron and Mortarion, of the World Eaters and Death Guard Legions, respectively, to the side of Chaos because of their own various personal grudges against the Emperor. Fulgrim of the Emperor's Children had been lured to the side of the Warmaster by the promise of power and personal perfection that the Chaos Gods, especially Slaanesh, offered to him and his vain Astartes. Lorgar of the Word Bearers, who had been responsible for the nascent rebellion and Horus's own corruption by Chaos, was also with the Warmaster. Three of the most loyal Legions who could not be swayed to the side of Chaos, the Dark Angels, Blood Angels and Ultramarines and their Primarchs, were sent on missions by the Warmaster far from Terra and the Istvaan System. The Imperial Fists and White Scars were too close to Terra to be contacted without raising suspicion, though Horus believed -- mistakenly -- that the White Scars' Primarch Jaghatai Khan, would ultimately take his side. Shortly before the Drop Site Massacre on Istvaan V, Fulgrim also attempted to sway his friend Ferrus Manus of the Iron Hands Legion to Horus' cause by using many of the same inducements that had been offered to the Adeptus Mechanicus, with whom the Iron Hands were closely allied in both temperament and philosophy. This attempt failed, and Fulgrim barely escaped with his life. Angered by the rebuff, Fulgrim promised he would deliver Manus' severed head to Horus in recompense, a promise he kept on Istvaan V. The Blood Angels were sent to the daemon-infested Signis Cluster and the Ultramarines to the world of Calth, where a large Word Bearers force, under First Captain Kor Phaeron, had massed to hold Roboute Guilliman's equally massive Legion in place while Horus made his play for Terra.

Of the other eventual Traitor Primarchs, Konrad Curze, the Night Haunter, was due to face disciplinary action from the Emperor which he did not believe he deserved; the Alpha Legion Primarch Alpharius had always been closer personally to his brother Horus than to his father the Emperor, although some evidence indicates that he and his twin brother Omegon's turn to Chaos was driven by mistaken loyalty to the Emperor; and the Iron Warriors' Primarch Perturabo's open and bitter rivalry with Rogal Dorn of the Imperial Fists and his feeling that he and his Legion were handed the worst tasks in the Great Crusade for which they never received the recognition they believed they were due made him an easy target for corruption.

The Thousand Sons had never planned to join Horus, but the path Tzeentch had mapped for that Legion and their potent psychic Primarch Magnus the Red ultimately led them to Chaos regardless. Unfortunately, the Space Wolves' unexpected assault on the Thousand Sons' homeworld -- a brutal campaign remembered as the Scouring of Prospero -- resulted in the destruction of the libraries of precious knowledge that Magnus and his fellow Thousand Sons held so dear. Mortally wounded by Leman Russ, Magnus fell to temptation as he watched Tizca, the capital city of Prospero and its famed libraries of ancient knowledge burn and he called out to the Chaos God Tzeentch to save both himself and the remains of his Legion. The God of Sorcery was only too happy to oblige and he transported Magnus and the Thousand Sons through the Warp to the Daemon World later known as the Planet of the Sorcerers. Magnus became a Daemon Prince of Tzeentch and now desired only vengeance against the Emperor for what he saw as a betrayal, never realizing that it was Horus who had truly engineered his downfall and corruption.

The remaining Space Marine Legions -- the Raven Guard, Salamanders, Iron Hands and Space Wolves -- remained staunchly loyal to the Emperor, though all but the Space Wolves would pay dearly for it in the battles to come. Beyond the Legions, Horus had already swayed Magos Regulus of the Adeptus Mechanicus to his side with promises of the Standard Template Construct (STC) databases of ancient technology recovered during the war with the Auretian Technocracy. This alliance delivered crucial Adeptus Mechanicus and Titan support to the Warmaster's Traitor Legion and Traitor Imperial Army forces.

Meanwhile, Horus and his confidante and mentor in the ways of Chaos, the Word Bearers' First Chaplain Erebus, conducted a ritual designed to communicate with the Chaotic entities of the Warp. They established contact with a daemon called Sarr'kell, who acted as an emissary of the Chaos Gods to Horus and the Traitor Legions. Horus was then deceived by the daemon into believing that the Chaos Gods had no interest in dominating the material universe, and were only lending their support so that Horus could overthrow the Emperor, who they claimed was creating devices that could destroy the daemonic beings of the Immaterium. Horus agreed, and promised to swear loyalty to Chaos Undivided after his fateful operations on Istvaan III.

Drop Site Massac​re
The Istvaan System’s third world, comfortably close enough to the Istvaanian sun to support human life, had become a virus-soaked mass grave marking the anger of Horus Lupercal in the aftermath of the Istvaan III Atrocity that marked the start of the Horus Heresy. The world’s population was nothing more than contaminated ash scattered over lifeless continents, while the bones of their cities remained as blackened smears of burnt stone – a civilisation reduced to memory in but a single day. The orbital bombardment from the Warmaster’s fleet, payloads composed of incendiary shells and virus-laden biological warfare pods, had seemingly spared nothing and no one anywhere on the world. Istvaan III lingered in silent orbit around its sun, almost grand in the extent of its absolute devastation, serving as the scarred tombstone for the death of an empire. With the destruction of the last surviving warriors on Istvaan III, the Traitor Legions of Horus made their way to Istvaan V, a flotilla of powerful warships and carriers bearing the martial pride of four Space Marine Legions, their ranks fully comprised of those whose loyalty was to Horus and Horus alone. Mass conveyors of Imperial Army units brought millions of armed men and their tanks and artillery pieces. Bloated Dark Mechanicum transports bore the Legio Mortis to Istvaan V, the dark Tech-priests ministering to the Dies Irae and its sister Titans as they prepared to unleash the unimaginable power of those war machines once more.

Final victory for the Traitors on Istvaan III had been bought with many lives, but in its wake the Traitor Legions were tempered in the crucible of combat to do what must be done to seize control of the Imperium. The process had been long and bloody, but the Warmaster's army was ready and eager to fight its brothers, where the lackeys of the Emperor would find their readiness to strike down their kith and kin untested. Such mercy would be their undoing, Horus promised. Horus would strike a vicious blow against the Imperium and the False Emperor -- a blow that would resonate down through the millennia for all time.

As Horus made the opening moves of his rebellion on Istvaan III, Ferrus Manus' oldest and dearest friend Fulgrim was ordered by the Warmaster to meet with the Iron Hands' Primarch aboard his flagship Fist of Iron in the hope that he could be swayed to the side of the Traitor Legions who now served Chaos. Fulgrim had sent the bulk of his III Legion and the 28th Expeditionary Fleet on to meet Horus and the 63rd Expeditionary Fleet in the Istvaan System while he and a small force aided the Iron Hands' 52nd Expeditionary Fleet in retaking the world of Callinedes IV from Orks. Great bonds of friendship and brotherhood had long existed between the two Legions, and Fulgrim felt that he could convince Ferrus of the righteousness of Horus' cause. Fulgrim's hope proved disastrously wrong and the meeting of the two Primarchs in Ferrus' private inner sanctum in his flagship's Anvilarium did not go well, as Ferrus was utterly outraged that his brothers would turn against their father the Emperor. The meeting ended in violence as The Gorgon made his difference of opinion over continued loyalty to the Emperor known to the Phoenician with his weapons, determined to stop Fulgrim's betrayal of the Imperium before it could begin.

The Warmaster was enraged at Fulgrim's failure at converting their brother Ferrus to their cause. Nevertheless, he still had to make preparations for the inevitable response of the Emperor, which was likely to arrive more quickly than anticipated and the Traitors needed to be prepared for it. Fulgrim was tasked to take a detail of Emperor's Children to the ruins of the alien fortresses that existed on Istvaan V and prepare that world for the final phase of the Istvaan operation. Though the Phoenician recoiled at the horrifying prospect of the menial role placed upon him, Horus explained that the Istvaan V phase of his plan was the most critical, and he could entrust this vital task to no other. Fulgrim supervised the vast teams of Dark Mechanicus earthmovers as they shifted the black sand of Istvaan V and formed a vast network of earthworks, trenches, bunkers and redoubts that stretched along the ridge of the Urgall Plateau. Laagers of anti-aircraft batteries were set up in the shadow of the walls, and mighty orbital torpedoes on mobile launch vehicles hid in the warrens of an ancient alien fortress. Fulgrim had set up his command within the remains of the keep and begun the work of ensuring that it would be a bastion worthy of the Warmaster. If the Emperor’s Legions wanted to destroy the Traitors, they were going to have to come down to the surface of Istvaan V to do so, as no obital bombardment would be powerful enough to dislodge the defenders or crack their resolve.

Overcome with mind-numbing rage at their brothers' treachery, Ferrus Manus and his Iron Hands Legion gratefully received the Emperor's orders through his brother Rogal Dorn. In response to Horus' betrayal of the Loyalist Astartes in the Sons of Horus, Emperor's Children, World Eaters and Death Guard Legions at Istvaan III, the Primarch of the Imperial Fists Legion, Rogal Dorn, on the direction of the Emperor who had learned of Horus' actions from the Loyalist survivors aboard the Eisenstein, ordered seven Loyalist Space Marine Legions to Horus' base on the world of Istvaan V to challenge the Warmaster's rebellion. They would attack in two waves and fall under the supreme command of the Iron Hands' Primarch Ferrus Manus. The Legions comprising the first wave were the Iron Hands, Salamanders, and the Raven Guard. The Legions comprising the second wave, who would arrive at Istvaan V after the first wave, were the Alpha Legion, Night Lords, Iron Warriors, and a large contingent of Word Bearers that their Primarch Lorgar had stationed in the star system. Unknown to Dorn and Ferrus Manus, the Night Lords, Alpha Legion, Iron Warriors and Word Bearers had all turned from their service to the Emperor and pledged their loyalty to Horus, and been instructed to keep their new allegiance to Chaos a secret.

Thousands of Drop Pods and Stormbirds were deployed for the initial assault. The first wave was under the overall command of the Primarch Ferrus Manus and besides his own X Legion, the Salamanders led by Vulkan, and the Raven Guard under the command of their Primarch Corax joined him. Vulkan's Legion assaulted the left flank of the Traitors' battle line while Ferrus Manus, the Iron Hands' First Captain Gabriel Santor, and 10 full companies of elite Morlocks Terminators charged straight into the centre of the enemy lines. Meanwhile, Corax's Legion hit the right flank of the enemy's position. The odds were considered equal; 30,000 Traitor Marines against 40,000 Loyalists. Horus was aware of the location of the Loyalists' chosen drop site and his troops fell upon the Loyalist Legions.

The battlefield of Istvaan V was a slaughterhouse of epic proportions. Treacherous warriors twisted by hatred fought their former brothers-in-arms in a conflict unparalleled in its bitterness. The mighty Titan war engines of the Machine God walked the planet’s surface and death followed in their wake. The blood of heroes and traitors flowed in rivers, and the hooded Hereteks Adepts of the Dark Mechanicum unleashed perversions of ancient technology stolen from the Auretian Technocracy to wreak bloody havoc amongst the Loyalists. All across the Urgall Depression, hundreds died with every passing second, the promise of inevitable death a pall of darkness that hung over every warrior. The Traitor forces held, but their line was bending beneath the fury of the first Loyalist assault. It would take only the smallest twists of fate for it to break. The forces on the surface have been embattled for almost three hours with no clear victor emerging. The loyalists waited for the second wave of 'allies' to make planetfall, believing they would be reinforced for their final advance. The traitors all knew their parts to play in this performance. They were all aware of the blood they must shed to spare their species from destruction, and install Horus as the Master of Mankind.

Though the Iron Hands, Raven Guard and Salamanders had managed to make a full combat drop and secured the drop site, the Urgall Depression, they did so at a heavy cost. Overwhelmed with rage, the headstrong Ferrus Manus disregarded the counsel of his brothers Corax and Vulkan and hurled himself against the fleeing rebels, seeking to bring Fulgrim to personal combat. His veteran troops -- comprising the majority of the X Legion's Terminators and Dreadnoughts -- followed. What had begun as a massed strike against the Traitors’ position was rapidly turning into one of the largest engagements of the entire Great Crusade. All told, over 60,000 Astartes warriors clashed on the dusky plains of Isstvan V. For all the wrong reasons, this battle was soon to go down in the annals of Imperial history as one of the most epic confrontations ever fought.

The Urgall Depression was churned to ruination beneath the boots and tank treads of countless thousands of Astartes warriors and their Legion’s armour divisions. The loyal primarchs could be found where the fighting was thickest: Corax of the Raven Guard, borne aloft on black wings bound to a fire-breathing flight pack; Lord Ferrus of the Iron Hands at the heart of the battlefield, his silver hands crushing any traitors that came within reach, while he pursued and dragged back those who sought to withdraw; and lastly, Vulkan of the Salamanders, armoured in overlapping artificer plating, thunder clapping from his warhammer as it pounded into yielding armour, shattering it like porcelain.

The traitorous Primarchs slew in mirror image to their brothers: Angron of the World Eaters hewing with wild abandon as he raked his chainaxes left and right, barely cognizant of who fell before him; Fulgrim of the lamentably-named Emperor’s Children, laughing as he deflected the clumsy sweeps of Iron Hands warriors, never stopping in his graceful movements for even a moment; Mortarion of the Death Guard, in disgusting echo of ancient Terran myth, harvesting life with each reaving sweep of his scythe.

And Horus, Warmaster of the Imperium, the brightest star and greatest of the Emperor’s sons. He stood watching the destruction while his Legions took to the field, their liege lord content in his fortress rising from the far edge of the ravine. Shielded and unseen by his brothers still waging war in the Emperor’s name. At last, above this maelstrom of grinding ceramite, booming tank cannons and chattering bolters – the gunships, drop-pods and assault landers of the second wave burned through the atmosphere on screaming thrusters. The sky fell dark with the weak sun eclipsed by ten thousand avian shadows, and the cheering roar sent up by the loyalists was loud enough to shake the air itself. The traitors, the bloodied and battered Legions loyal to Horus, fell into a fighting withdrawal without hesitation.

The second wave of "Loyalist" Space Marine Legions descended upon the landing zone on the northern edge of the Urgall Depression. Hundreds of Stormbirds and Thunderhawks roared towards the surface, their armoured hulls gleaming as the power of another four Astartes Legions arrived on Istvaan V. Yet the Space Marine Legions of the reserve were no longer loyal to the Emperor, having already secretly sworn themselves to Chaos and the cause of Horus. The Night Lords of Konrad Curze, the Iron Warriors of Perturabo, the Word Bearers of Lorgar Aurelian, and the Alpha Legion of Alpharius represented a force larger than that which had first begun the assault on Istvaan V. The secret Traitor Legions mustered in the landing zone, armed and ready for battle, unbloodied and fresh.

The Iron Warriors had claimed the highest ground, taking the loyalist landing site with all the appearance of reinforcing it through the erection of prefabricated plasteel bunkers. Bulk landers dropped the battlefield architecture: dense metal frames fell from the cargo claws of carrier ships at low altitude, and as the platforms crashed and embedded themselves in the ground, the craftsmen-warriors of the IV Legion worked, affixed, bolted and constructed them into hastily-rising firebases. Turrets rose from their protective housing in the hundreds, while hordes of lobotomised servitors trundled from the holds of Iron Warriors troopships, single-minded in their intent to link with the weapons systems’ interfaces. The Word Bearers bolstered their brother Legions on one flank of the Urgall Depression while the Night Lords took positions on the opposite side. Down the line, past the mounting masses of Iron Warriors battle tanks and assembling Astartes, First Captain Sevatar of the Night Lords and his First Company elite, the Atramentar took up defensive positions. Both the Word Bearers and the Night Lords were to be the anvil, while the Iron Warriors would be the hammer yet to fall. The enemy would stagger back to them, exhausted, clutching empty bolters and broken blades, believing their presence to be a reprieve. Dragging their wounded and dead behind them, Corax and Vulkan led their forces back to the drop site to regroup and to allow the warriors of their recently arrived brother Primarchs of the second wave a measure of the glory in defeating Horus. Though they voxed hails requesting medical aid and supply, the line of Astartes atop the northern ridge remained grimly silent as the exhausted warriors of the Raven Guard and Salamanders came to within a hundred metres of their allies. It was then that Horus revealed his perfidy and sprung his lethal trap. Inside the black fortress where Horus had made his lair, a lone flare shot skyward, exploding in a hellish red glow that lit the battlefield below. The fire of betrayal roared from the barrels of a thousand guns, as the second wave of Astartes revealed where their true loyalties now lay. Ferrus Manus looked on in stunned horror as Fulgrim laughed at the look on his brother's face as the forces of his "allies" opened fire upon the Salamanders and Raven Guard, killing hundreds in the fury of the first few moments, hundreds more in the seconds following, as volley after volley of Bolter fire and missiles scythed through their unsuspecting ranks. Even as terrifying carnage was being wreaked upon the Loyalists below, the retreating forces of the Warmaster turned and brought their weapons to bear on the enemy warriors within their midst. Hundreds of World Eaters, Sons of Horus and the Death Guard fell upon the veteran companies of the Iron Hands, and though the warriors of the X Legion continued to fight gallantly, they were hopelessly outnumbered and would soon be hacked to pieces. The Iron Hands had damned themselves by remaining in the field.

The Raven Guard front ranks went down as if scythed, harvested in a spilling line of detonating bolter shells, shattered armour and puffs of bloody mist. Black-armoured Astartes tumbled to their hands and knees, only to be cut down by the sustained volley, finishing those who fell beneath the initial storm of head- and chest-shots. Seconds after the first chatter of bolters, beams of achingly bright laser slashed from behind the Word Bearers as the cannon mounts of Land Raiders, Predators and defensive bastion turrets gouged through the Raven Guard and the ground they stood upon. The Iron Warriors and Word Bearers kept reloading, opening fire again, hurling grenades and prepared to fall back. The Word Bearers Legion had taken up landing positions on the west of the field, ready to sweep down and engage the Raven Guard from the flank. The Raven Guard were confronted by the treacherous Word Bearers, with their Primarch Lorgar, the First Captain Kor Phaeron and the First Chaplain Erebus at their vanguard. The two Legions fought one another in bitter combat. In the midst of this battle, the Word Bearers unleashed the elite unit known as the Gal Vorbak -- Astartes who had allowed themselves to be possessed by daemons. They attacked the Raven Guard's Primarch en masse, but despite the advantage of their numbers, Corax's formidable abilities as a consummate warrior proved to be more than a match for the possessed Astartes, and he slew them with impunity. Seeing the slaughter of his most favoured sons, Lorgar intervened and prevented the death of the remaining Gal Vorbak Astartes. The two opposing Primarchs then duelled one another in close combat, and the Raven Guard's Primarch quickly gained the upper hand over his outmatched brother. Lorgar had always been more of a scholar than a warrior and Corax prepared to execute him for his betrayal of the Emperor. Lorgar was spared from execution by the intervention of the Night Lords' Primarch, Konrad Curze, at the last moment. The Night Haunter and the Raven fought a brutal melee. Curze quickly gained the upper hand over his battle-weary brother and prepared to slay him, but Corax managed to escape death by taking to the sky with his master-crafted Jump Pack.

The outnumbered Loyalists were then surrounded and brutally butchered. Refusing to surrender, the remaining Raven Guard and Salamanders Astartes stubbornly defended themselves, trying to hold off the inevitable slaughter for as long as possible. Though they suffered an atrocious number of casualties, the Loyalists managed to hold their own, until the Primarchs Mortarion of the Death Guard and Angron of the World Eaters joined the fray. Bolstered by the support of the infamous Imperator-class Titan Dies Irae, the Traitors killed tens of thousands of Loyalist Astartes. At the height of the massacre the Warmaster Horus entered the fray, at the head of the elite Sons of Horus Terminators known as the Justaerin, slaughtering the Loyalists in wrathful anger.

Any hope for escape for the Loyalists was quickly crushed when the traitorous Iron Warriors destroyed the first wave's drop ships. The Loyalist starships still orbiting the embattled planet were also largely annihilated by the vastly superior numbers of the Traitor's fleet. Despite the odds arrayed against them, some of the Loyalists on the ground managed to survive against these odds—they miraculously escaped through the tightening cordon of Traitors that surrounded their position. The Raven Guard fared better than the Salamanders in escaping the brutal massacre. But the Salamanders managed to assist a few surviving Astartes from the decimated Iron Hands Legion to also escape the slaughter. Imperial history does not record the fate of these surviving Salamanders or their missing Primarch Vulkan. The Raven Guard's Primarch just barely managed to board a fleeing Thunderhawk gunship to make good his escape, but was thwarted in the attempt when it was shot down almost immediately by the gunfire of the Traitors. The badly damaged ship crashed on the outskirts of the Urgall Plateau.

Horus Trium​phant
"The road to Terra is open. The time has come for us to take the war to the Emperor in his most impregnable fastness! We will make immediate preparation for the invasion of Terra and an assault on the Imperial Palace. Make no mistake, and it will be ours, my brothers! This will be no easy task, for the Emperor and his deluded followers will fight hard to prevent us from interfering with his plans for godhood. Doubtless much blood has yet to be spilled, theirs and our own, but the prize is the galaxy itself...Are you with me?"

- Warmaster Horus, Master of Istvaan

After the killing had stopped and the dead were gathered into great funeral pyres across the broken desert of the Urgall Depression, the once-grey skies of the planet burned orange with the reflected glow of a thousand pyres. The firelight bathed the rippling, glassy sands in a warm radiance, and towering pillars of black smoke from the burning corpses filled the air. Thousands of Astartes loyal to Horus gathered before a great reviewing stand, constructed by the Tech-priests of the Dark Mechanicus with astonishing speed. As the sun began to sink beyond the horizon, the smooth black planes of the stand shone with a blood red glow. The stand was erected as a series of cylinders of ever decreasing diameter, one standing atop another. The base was perhaps a thousand metres in width, constructed as a great grandstand upon which the Sons of Horus stood, their pre-eminent position as the elite of the Warmaster in no doubt after this great victory. Each warrior bore a flaming brand, and the firelight cast brilliant reflections from their armour.

Atop this pedestal of flame was another platform, occupied by the senior officers of the XVI Legion. Above the senior officers of the Sons of Horus stood the Traitor Primarchs. The sheer magnificence of such a gathering of might was breathtaking. Seven beings of monumental power stood on the penultimate tier of the reviewing stand, their armour still stained with the blood of their foes, their cloaks billowing in the winds that swept the Urgall Depression. Finally, the uppermost tier of the reviewing stand was a tall cylinder of crimson that stood a hundred metres above the Primarchs. Horus stood on top of it, his clawed gauntlets raised in salute. A furred cloak of some great beast hung from his shoulders, and the light of the corpse pyres reflected from the amber eye upon his breastplate. The Warmaster was illuminated from below by a hidden light source, bathing him in a red glow that gave him the appearance of the statue of a legendary hero, as he stood looking down on the endless sea of his followers from the towering platform. As the sun finally dipped below the horizon, a flight of assault craft roared over the Urgall Hills, their wings dipping in salute to the mighty warrior below. Solid waves of cheering crashed against the reviewing stand, howls of adulation torn from tens of thousands of throats. No sooner had the aircraft passed overhead than the massed Astartes began to march around the reviewing stand, their arms snapping out and hammering their breastplates in salute of the Warmaster. At some unseen signal a flame ignited on the northern slopes of the Urgall Depression and a blazing line of phosphor leapt across the ground in a snaking arc that described the outline of an enormous blazing eye upon the hillside. The adulation soared to new heights as the Eye of Horus seared itself into the sands of Isstvan V, the Warmaster’s forces roaring themselves hoarse in his praise. Super-heavy tanks fired in salute of Horus, and the towering immensity of the Dies Irae inclined its massive head in a gesture of respect. The ashes of the dead fell like confetti over Horus' mighty army as thousands of Traitor Astartes cheered, their cries of "Hail Horus! Hail Horus!" resounding long into the darkness.

Barely a handful of Loyalist Space Marines escaped with their lives from Istvaan V to bring dreadful word of the further betrayal of four more Space Marine Legions to the Emperor. A critically wounded Corax made the dangerous journey through the Immaterium back to Terra, arriving 133 days after departing the Istvaan System and finally reaching the Sol System -- the heart of the Imperium -- to seek audience with the Emperor. Vulkan was missing and presumed dead, though he would later reemerge after a harrowing journey back to Terra himself, to lead his Legion once more. The Salamanders, along with the Iron Hands and the Raven Guard, would spend the remainder of the Horus Heresy rebuilding their decimated Legions and were too weakened to play any further role in the great conflict.

In the days after the battle, the Traitor Legions salvaged a large number of vehicles, wargear and other war materiel from what the Loyalist Legions had left on the field. This salvage was repaired and modified for the Traitor Legions' use and then put back into frontline service to be used against the Imperium. Some of this equipment would still be in service with certain Chaos Space Marine warbands in the late 41st Millennium. Orbital space around Istvaan V was busy as the vessels of 8 Legions assumed formation prior to transit to the system jump point. Over 3,000 vessels jostled for position above the darkened fifth planet, their holds bursting with warriors sworn to the service of Horus. Tanks and monstrous war machines had been lifted from the planet with incredible efficiency and an armada greater than any in the history of the Great Crusade assembled to take the fire of war into the very heart of the Imperium.

Following the victory of the Drop Site Massacre, Horus called for a conclave of the Primarchs of all 8 of the Traitor Legions aboard his flagship, the Vengeful Spirit. Five of the Primarchs, including four who had fought at Istvaan V, met in person, including Horus, Fulgrim, Angron, Mortarion and Lorgar. Three appeared through the use of hololithic emitters that transmitted their signals through the Warp, including Perturabo, the Night Haunter and Magnus the Red, who had only recently joined the Traitors after the Scouring of Prospero when the broken remains of his XV Legion had been transported by Tzeentch into the Eye of Terror to the Planet of the Sorcerers. The Thousand Sons, bitter at what they perceived as their betrayal by the Emperor, now willingly became the eighth Traitor Legion. The council of Traitor Primarchs made their plans for the next step in their war against the Emperor and then each Legion went its way according to its assigned role.

The fleets of Angron, Fulgrim, Mortarion, Lorgar and Horus' own Legion would rendezvous at Mars, now that word had come from the Tech-priest Regulus, the Mechanicus' liaison with the 63rd Expeditionary Fleet, of that planet’s fall to Horus’ supporters within the Mechanicus during the internecine conflict known as the Schism of Mars. With the manufacturing facilities of Mondus Gamma and Mondus Occullum wrested from the control of the Emperor’s forces, the forges of Mars were free to supply the Warmaster’s army. The eager warriors of the Alpha Legion were singled out by Horus for a vital mission, one upon which the success of the entire venture could depend. Following Horus' manipulation of Leman Russ into assaulting the homeworld of the Thousand Sons, the Space Wolves were known to be operating in the region of Prospero. In the nearby system of Chondax, the White Scars of Jaghatai Khan were sure to have received word of Horus’ rebellion and would no doubt attempt to link up with the Space Wolves. Horus could not allow such a grave threat to appear, and so the warriors of Alpharius were to seek out and attack these Legions before they could join forces.

The Night Haunter’s fleet had already departed, bound for the planet of Tsagualsa, a remote world in the Eastern Fringe that lay shrouded in the shadow of a great asteroid belt. From there, the Night Lords’ terror troops would begin a campaign of genocide against the Imperial strongholds of Heroldar and Thramas, star systems that, if not taken, would leave the flanks of the Warmaster’s strike on Terra vulnerable to attack. The Thramas System was of particular importance, as it comprised a number of Mechanicus Forge Worlds whose loyalty was still to the Emperor. This campaign would also serve to tie up the dreaded Dark Angels Legion, so that the forces of the Lion wouldn't be brought to bare against Horus and his upcoming campaign against Terra.

The ships of the Iron Warriors prepared to make the journey to the Phall System where a large fleet of Imperial Fists vessels were known to be regrouping after a failed attempt to reach Istvaan V in time to join the Loyalist assault. Though Rogal Dorn’s warriors had played no part in the Drop Site Massacre, Horus could not allow such a powerful Loyalist force to remain unmolested. The enmity between bitter Perturabo and proud Dorn was well known, and it was with great relish that the Iron Warriors set off to do battle with their old rivals. With his flanks covered and the Space Marine forces that could potentially reinforce the heart of the Imperium soon to be embroiled in war, the Traitors were ready to unleash seven Terran years of devastating civil war upon the Imperium in the name of Horus and the Dark Gods.

Execution Team
After learning of Horus' perfidy on Istvaan III, the Sires and Siresses of the various secret Imperial Assassin Clades were tasked by Malcador the Sigillite, the secret Master of Assassins, with the daunting task of slaying the Arch-Traitor Horus, for if they could accomplish this monumental undertaking, they could effectively crush the nascent rebellion against the Emperor before it could inflict further damage upon His Imperium. But every assassination attempt against the Warmaster had failed thus far. Though the operatives sent by the Clades were their finest students and equal to the task, every attempt had resulted in failure. The Assassins had thrown their most gifted students into a meat-grinder, sending them in blind and half-prepared. Every strike against Horus was broken, and he had shrugged off each attempt without notice. Every time the Clade masters met, they were forced to grimly listen to a catalogue of each other's failures.

After the last failed attempt by Clade Venenum, a new strategy was decided upon. With the advice of a special guest in the form of Constantin Valdor, the Captain-General of the Legio Custodes who served as the personal protectors of the Emperor, the masters of the Clades realised that their mission plans were not flawed, they were simply not enough. No single Assassin, no matter how well-trained, no matter which Clade they hailed from, could ever hope to terminate the Arch-Traitor alone. But a collective of killers, a strike team consisting of an elite unit of killers hand-picked for the task, might be enough to succeed. There had never been a precedent for such an initiative, for the Emperor would have never sanctioned assassination as an official Imperial policy. The deployment of an Assassin was a delicate matter and never one taken lightly. In the past, the Clades had fielded two or three of their operatives on a single mission when the circumstances were most extreme, but these Assassins were always drawn from the same Clade, and even this occurred only after much deliberation. Yet in this crucial and extreme instance, Malcador, serving as Director Primus of the Clades, decided that more drastic measures were needed to accomplish the Assassins singular goal. He authorised the creation of the first Imperial Execution Force.

Horus did not adhere to the rules of war, nor did he baulk at the use of a tactic because it offended sensibilities. At Istvaan III he had bombed his sworn brethren, his own warriors even, into obliteration. Nothing, no mater how vile, was beyond him. It was decided by the Assassins that if they were to kill this foe, they could not limit themselves to the moral abstracts that had guided the Clades in the past. They had to dare to exceed them. And so a select strike team of Assassins was formed. An Execution Force, the first of its kind. Six Assassins, one from each Clade, were gathered together and given the select task of killing Horus by any means necessary. In the meantime, the Dark Apostle Erebus had decided on a bold course of action of his own. He firmly believed that as long as the Traitor Legions followed Horus, all would be as it should and as the Dark Gods had promised. Victory would come soon enough, perhaps even sooner than any of them might expect, for after the latest assassination attempt on the Warmaster, Erebus had come to realise a truism of warfare: if a tactic could be used against Horus, then it could also be used by the Traitors against the Emperor.

"Black Pariah"
Within Imperial history, only one "Black Pariah" has ever existed. He was a former Imperial Assassin by the code-name of "Spear." Born as a human Untouchable, he was captured by the Silent Sisterhood and brought to Terra, where the Clade Culexus experimented upon and augmented him in an attempt to create a more powerful and deadly form of Culexus Assassin. It is not known whether these augmentations or his unnatural abilities made him a "Black Pariah." Spear was eventually deemed too unstable and dangerous by his Clade's masters to be left alive. He was placed in the care of the Sisters of Silence and was sent aboard one of their lone vessels, bound for the heart of a nearby sun. Unfortunately, this vessel was intercepted by a Traitor vessel carrying the Dark Apostle Erebus of the Word Bearers Legion. Boarding the Sisters' vessel, the Word Bearers killed all aboard, with the exception of Spear.

Sensing the usefulness of such a unique specimen, Erebus found a new purpose for his captive. He forced Spear to undergo a painful and vile Chaotic ritual, in which a minor daemon from the Immaterium was bonded with the former Imperial Assassin. This bonding created a highly dangerous apex predator -- a "counter-psyker" -- capable of redirecting a psyker's attack directly back upon him. In order to utilise this ability, the "Black Pariah" first had to obtain a sample of his target's blood. This was a necessary component that helped him synchronise with his target's psionic abilities in order to reflect their attacks. Two standard years later, following the events of the Drop Site Massacre on Istvaan V, Erebus tasked his deadly minion to assassinate the Emperor. Spear spent an inordinate amount of time in order to painstakingly reach his ultimate goal -- a document that possessed a minute drop of the Emperor of Mankind's precious blood. Spear eventually obtained this document upon the world of Dagonet, which would bring him into direct conflict with the Imperial Execution Force.

Dagonet
As the Horus Heresy progressed and word filtered throughout the galaxy of Horus' galactic uprising, numerous worlds began to erupt into anarchy as the populations began to split over whether they should remain loyal to the Emperor or join the Warmaster. Dagonet was one such world, where Horus Lupercal was second only to the Emperor in being celebrated by the people of the planet; statues in Horus' honour were raised everywhere, and the Dragoneti spoke of him as "the Liberator." As the historic record went, in the early years of the Great Crusade, Dagonet had languished under the heel of a corrupt and venal priest-king who ruled the planet through fear and superstition. Horus, at the head of his Luna Wolves Legion, had come to Dagonet and freed the world -- accomplishing the deed with only one round of ammunition expended, the single shot he fired that dispatched the tyrannical priest-king. The victory was one of the Warmaster's most celebrated triumphs, and it ensured he would be revered forever as Dagonet's saviour. The Dagoneti clans had started the uprising against the Imperium when the Heresy began. Dagonet's Imperial Governor issued a formal statement of support for the cause of Horus. The world's nobles had declared in favour of Horus and rejected the rule of Terra. The common people were the ones fighting back in the name of the Emperor. There was blood in the streets of the Dagoneti capital city as soldiers fought soldiers and militia fought clan guards. Those who could flee the star system filled every starship they could get their hands on. It was small wonder that the aristocratic clans who now ruled the planet would give their banners to Horus instead of a distant Emperor who had never set foot on their world.

The Execution Force soon learned the future whereabouts of where Horus would be. Agents of the Imperium operating covertly in the Taebian Sector report a strong likelihood that Horus was planning to bring his flagship, the Vengeful Spirit, to the planet Dagonet in order to show his flag. The Clades believed that the Warmaster’s forces would use Dagonet as a foothold from which to secure the allegiance of every planet in the Taebian Stars Sector. Dagonet was a keystone world in the politico-economic structure of the Taebian Sector, and if it fell fully under the shadow of Horus, then it would mark the beginning of a domino effect, as planet after planet along the same trade axis followed suit. Every Loyalist foothold in this sector of space would be in jeopardy. One Imperial vessel would be able to slip through the Warp to Dagonet, far easier than an entire reprisal fleet. Six Assassins, the best of their Clades, could bring death. The Execution Force would embed on Dagonet and set up multiple lines of attack. When Horus arrived there, they would terminate his command with extreme prejudice. Horus' assassination at this juncture would throw the Traitor forces into disarray and break the rebellion before it could advance on to the Segmentum Solar.

The Execution Force successfully circumvented all detection and was able to secretly arrive upon Dagonet. The Execution Force gathered intelligence to determine what exactly had occurred on Dagonet. In the first moments of the insurrection, desperate signals had been sent to the Space Marine Legions and their Legion fleets; but these had gone unanswered. Both the starships of the admiralty and the Legions had battles of their own to fight, far from the Taebian Stars. They would not intervene. For all the fire and destruction the collapse of Dagonet and its sister worlds might cause, there were larger conflicts being addressed; no crusade of heroes was coming to ride to the rescue. The civil war on Dagonet was a rout, and it was those who stood in the Emperor's name who were dying. Across the planet, the forces that carried Horus' banner were only days away from breaking the back of any resistance. Dagonet was already lost. The turncoat nobility on this planet did not need to see Horus to adhere to his banner. His influence hung over Dagonet like an eclipse blotting out the sun. They were fighting in his name in fear of him, and that was enough. And when the Traitors finally won, Horus' work would be done for him. This same thing was happening all across the galaxy, on every world too far from the Emperor and the rule of Terra. When Dagonet fell, Horus would turn his face from this place and move on, his advance one step closer to the gates of the Imperial Palace.

While gathering intelligence and deciding upon the best course of action, two of the Execution Force's number decided to set about on a different course of action. The Venenum Assassin Jenniker Solam had become distracted by her mission with the plight of the local Loyalist Dagoneti who continued to wage their desperate war against the pro-Horus planet's nobility. The Dagoneti introduced her to the forbidden writings of the Lectitio Divinitatus which postulated the worship of the Emperor of Mankind as a divine being, the one, true God of humanity. Solam became a willing convert of this nascent religious movement and vowed to help the Dagoneti with their plight. Her interest in helping the people of Dagonet and her new spirituality created friction with the rest of the Execution Force, and so she took her leave, and the mission to assassinate Horus continued on without her. The Culexus Assassin Iota, who showed great interest in Solam's quest, followed her. The pair of Assassins soon came into conflict with the "Black Pariah" known as Spear. Realising the dire threat of such a creature, the two Assassins attacked the Traitor Assassin. In the ensuing battle Solam was mortally wounded. Iota utilised her Animus Speculum, unleashing her innate anti-psyker abilities upon Spear, seriously wounding him in the process. Though Iota finally gained the upper hand, her efforts were for naught, as Spear was able to sample a drop of her spilt blood. This enabled the Traitor Assassin to engage his genetic lock, using his own innate abilities to reflect Iota's attack back onto her, boiling the Assassin in the crucible of her own powers.

The dying Solam was found by her brother Eristede Kell, a Vindicare Assassin and the Execution Force's team leader. Solam made Kell promise to kill the Traitor Assassin, not out of vengeance, but for the sake of the God-Emperor. The Execution Force managed to salve Iota's memory coil from her Animus Speculum and review her confrontation with the creature known as Spear. Realising the dire threat that this Counter-Assassin represented, the Assassins' mission became twofold; assassinate Horus and kill the creature that had murdered their comrade.

Moment of Truth
The Sons of Horus Legion finally arrived in-system at Dagonet. The Vengeful Spirit settled in orbit above the world of Dagonet. The vessel had brought a military force of such deadly intent and utter lethality that the planet and its people had never known the like, in all their recorded history. And it was only the first. Other warships were following close behind. This was the visitation granted to Dagonet by the Sons of Horus, the tip of a sword blade forged from shock and awe. Far below, on the planet's surface, across the white marble of Liberation Plaza, a respectful hush fell over the throng of people who had gathered. A pregnant hush fell over the Dagoneti, as they looked to the sky and awaited the arrival of their redeemer, the owner of their new allegiance. Their war-god, the Warmaster Horus. At this time, the Execution Force was in place. The Vindicare Assassin Kell waited in the perfect assassin's perch, ready for the Warmaster's arrival. The Callidus passed a measuring gaze over the nervous lines of Dagoneti Planetary Defence Force soldiers and the robed nobles standing back on the gleaming, sunlit steps of the great hall. Governor Nicran was there among them, waiting with every other Dagoneti for the storm that was about to break. Suddenly, there was a blast of fanfare from the trumpets of a military band, and Governor Nicran stepped forward. When he spoke, a Vox-bead at his throat amplified his voice. "Glory to the Liberator!" he cried. "Glory to the Warmaster! Glory to Horus!" The assembled crowd raised their voices in a thundering echo. The Sons of Horus teleported onto the planet's surface. The tallest of the superhuman warriors, his battle gear decked with more finery than the others, stepped forward. He was covered with honour-chains and combat laurels, and about his shoulders he wore a metal dolman made from ores mined in the depths of Cthonia; the Mantle of the Warmaster, forged by Horus' captains as a symbol of his might and unbreakable will. He drew a gold-chased Bolt Pistol, raising it up high above his head; and then he fired a single shot into the air, the round crashing like thunder. The same sound that rang about Dagonet on the day they were liberated. Before the empty shell casing could strike the marble at his feet, the crowd were shouting their fealty. Glory to Horus! The towering warrior holstered his gun and unsealed his helmet, drawing it up so the world might see his face. This was the moment of truth. The Vindicare Assassin placed his crosshairs on the centre of the scowling grille of the Warmaster's helmet. There was no hesitation, no margin for error. The Assassin fired his Exitus Rifle. The shot struck the target in the throat, reducing the flesh to atoms, superheating the fluids into steam, boiling skin and vapourising bone. The only sound was the fall of the headless corpse as it crashed to the ground, blood jetting across the white marble and the Warmaster's shining mantle. Horus was dead.

But the Assassins of the Execution Force had been duped. Horus had sent a surrogate, a sacrificial proxy. The Vindicare had killed Luc Sedirae, the Captain of the Sons of Horus' 13th Company. Though the warrior Kell shot wore the mantle of the Warmaster, the unique robe belonging to the Primarch himself, it had all been a ruse. In their anger, the Sons of Horus turned upon the populace of Dagonet and began to slaughter them in earnest.

Amidst the chaos and anarchy of the massacre of the entire populace of Dagonet, Kell finally tracked Spear down and with the assistance and sacrifice of his fellow Assassin, managed to finally draw out and kill the "Black Pariah." Kell found himself was the lone survivor of the first Imperial Execution Force. The Vindicare Assassin departed Dagonet and decided to make one last attempt on the Warmaster's life. He made a suicidal attack on Horus' flagship. Kell set his own ship on a direct course for the Vengeful Spirit's command bridge, where he seemed to meet his own fate when he ejected himself into space once he reached his intended target, in an ultimately vain attempt to land a desperate shot at Horus while he stood looking out of the Battle-Barge bridge's armoured observation deck into space. The Execution Force's assassination attempt had failed.

In the aftermath of the events on Dagonet, Horus confronted Erebus within his private chambers. He chastised the Dark Apostle for his audacious plan to assassinate the Emperor, declaring that when the opportune moment finally dawned, it would be him -- and him alone -- who killed the Master of Mankind.

Battle of Calth
While Horus made his plans for what would become known as the infamous Drop Site Massacre on Istvaan V, the Warmaster sent word to the XVII Legion’s Primarch Lorgar that the time had come for his Astartes, the Word Bearers, to strike against the Imperium. The Warmaster was keenly aware of the bitter hatred that Lorgar had for his Primarch brother Roboute Guilliman and his XIII Legion, the Ultramarines, who had once humiliated the Word Bearers by destroying their city of Monarchia on the world of Khur at the Emperor's orders during the Great Crusade. The Ultramarines had taken no pleasure in this act, which was intended to teach Lorgar and his Astartes to adhere to the atheistic doctrines of the Imperial Truth rather than spread the false belief that the Emperor was divine to all the worlds that they conquered. Yet Lorgar and the Word Bearers had never forgiven the Ultramarines for this action and they longed for vengeance against the XIII Legion.

Horus told Lorgar that he had fed Guilliman false intelligence in regard to a possible threat within the Veridian System in the Segmentum Tempestus, far to the galactic south of Terra. This supposed threat stemmed from the Orks of the Ghaslakh Empire. Horus had ordered the XIII and the XVII Legions to muster and meet at the world of Calth in the Ultramarines' Realm of Ultramar, in order to conduct a massive joint campaign of extermination against the Ghaslakh xenohold, a common mission for the Astartes during the final days of the Great Crusade.

It would be at Calth that Lorgar would launch a surprise attack on the Ultramarines whilst they were gathered for the campaign against the Orks of Ghaslakh. The XIII Legion would be caught completely unaware while the Word Bearers would use the advantage of surprise to completely annihilate their hated rivals. The assault at Calth would also allow the Word Bearers to reveal that they, too, now served Ruinous Powers. Calth was not chosen as the site of the confrontation between the Word Bearers and the Ultramarines by chance, for the Word Bearers intended to destroy one of the jewels in the Ultramarines' realm of Ultramar (then known as the Ultramar Coalition), just as the XIII Legion had destroyed one of the Word Bearers' greatest achievements, the sacred city of Monarchia, four decades earlier.

Horus ordered the majority of the XVII Legion to Ultramar, and the dark powers of the Warp gave them sure and swift passage across the increasingly restless Immaterium. As the Word Bearers entered Ultramar space, Lorgar prepared his Legion for the inevitable slaughter that would follow. Command of the main assault force was given to Kor Phaeron, the First Captain of the XVII Legion and one of Lorgar’s most favoured champions. Calth was to be Kor Phaeron’s operation to execute, far more than it was Lorgar’s. Kor Phaeron had planned the assault of Calth for his Primarch meticulously, and executed it with the aid of the Dark Apostle Erebus. The punishment and annihilation of the XIII Legion was the campaign's principal aim; the humiliation and execution of Lorgar's hated rival Roboute Guilliman was a secondary objective. But for Lorgar, the assault would also mark his first opportunity to gain true favour in the eyes of the Dark Gods he now served; to prove to them that he had earned his place as their Chosen One.

The Word Bearers turned Calth's own orbital defence platforms on the Veridian star, stripping away the outer layers of its photosphere and destabilising it, ultimately rendering the surface of Calth uninhabitable. At the same time, the Word Bearers had used the battle taking place on Calth to summon a massive Warp Storm called the Ruinstorm, that was intended to cut off Ultramar from the rest of the galaxy and prevent the Ultramarines from providing any reinforcements to Terra as Horus made his assault upon humanity's homeworld. The eruption of the Ruinstorm cut off Calth from the main body of the Ultramarines Legion and left the Astartes of the XIII Legion trapped on Calth locked in a brutal subterranean war with those Word Bearers units that had also been left behind when their Legion retreated from the Viridian System. Yet, despite the Loyalists' last-minute victory and the survival of the Ultramarines Legion and its Primarch, the Forces of Chaos could consider their assault on Calth a success. The XIII Legion had been badly crippled and no longer presented a viable threat to Horus' plan to drive on Terra. Erebus had managed to complete his blasphemous ritual on Calth's surface, which summoned a Ruinstorm to the galaxy's Eastern Fringe -- a monstrous Warp Storm larger and more destructive than anything space-faring humanity had witnessed since the days of the Age of Strife. It would split the void asunder, dividing the galaxy in two and rendering vast tracts of the Imperium impassable for centuries. The Ruinstorm would also isolate and trap those Loyalist forces caught behind it like the Ultramarines, preventing them from coordinating their efforts and supporting one another as the Traitor Legions moved towards Terra. It would even prevent them from warning each other, for a time, of the Warmaster's betrayal and the civil war that had begun to consume the Imperium. The Ruinstorm would leave Terra alone in the void, infinitely vulnerable to the approaching shadow of Horus.

Signus Prime
When the Imperial Warmaster Horus, the greatest of the Emperor's genetic sons, fell to the temptations of the Chaos Gods and became swayed by their promises of power, he sought to sway his brother Primarchs to his cause. Horus and Sanguinius, the Primarch of the Blood Angels Legion, had fought many campaigns by each other's side. Their relationship was so close it had even incited jealousy amongst their brother Primarchs on occasion. But in his black heart Horus knew that Sanguinius would never willingly betray their father, and so he had formulated an audacious plan to either convert the Blood Angels Legion to his cause or utterly destroy them. To this end, Horus had discovered decades earlier a carefully guarded secret of the Blood Angels when he fought alongside Sanguinius' IX Legion in a xenocidal campaign on the world of Melchior. Horus had come upon his brother Primarch in a sunken ruin of an alien chapel and had witnessed the unthinkable -- Sanguinius murdering one of his own Astartes. Sanguinius explained his actions to his bewildered brother. He had discovered that within his own genome there was a trait that lay buried and waiting to be awakened. This genetic flaw would later be known as the Red Thirst.

Sanguinius had been aware of the flaw in his genome for several years, keeping the truth from the Emperor and his fellow Primarchs. Some of the Angel’s sons had learned a measure of the truth, but only Azkaellon, First Captain Raldoron, the IX Legion’s Master Apothecary on the Legion homeworld of Baal  and a few others were fully aware of the extent of this affliction. They were united with Sanguinius in finding a way to repair this Flaw. Horus swore to his brother that he would never speak of this matter to anyone, even their father. He would keep this promise for as long as Sanguinius wished him to. The Angel was touched by his brother's gesture and expressed his gratitude. Horus solemnly vowed to help Sanguinius deal with this matter, however long it took. Little did they know at the time, that one day a corrupted Horus would take full advantage of this knowledge and attempt to turn the Blood Angels' Flaw against them. Decades later, Horus exploited this knowledge of the Blood Angels' genetic Flaw. The Warmaster had found a way to sway his "beloved" brother's Legion to his cause and the service of the Chaos Gods. In his capacity as Imperial Warmaster, Horus ordered Sanguinus to gather his entire Legion and make for the Signus Cluster, a triple star system located in the Ultima Segmentum near the Eastern Fringe. His IX Legion was to cleanse the 7 worlds and 15 moons that comprised the Signus Cluster of xenos invaders and release the humans settled there from their xenos-overlords. To further entice Sanguinius, the Warmaster informed him that he had found the means by which the Blood Angels would be able to excise the darkness from within their souls, and rid themselves of the Flaw. If Sanguinius obeyed the Warmaster's command in this matter, then Horus promised him that the Blood Angels would find a new freedom. Sanguinius had no reason to doubt Horus, for they were as close as two brothers could be. Many of their fellow Primarchs were even jealous of the closeness between the pair. Sanguinius relished the opportunity to once again prove the value of their bond, and so the Blood Angels Legion gathered in its entirety and duly set course for the Signus Cluster, unaware that they were heading into a terrible trap.

Unaware of the Warmaster's perfidy, Sanguinius willingly obeyed his brother Primarch and immediately set out for this volatile region of space. Unbeknownst to the Blood Angels, they were blindly walking into a deadly trap, for the Signus Cluster had fallen prey to agents of the Ruinous Powers and become a veritable Realm of Chaos -- a system of hellish Daemon Worlds under the rule of a Greater Daemon of Slaanesh known as Kyriss the Perverse. When the Blood Angels arrived in-system, their fleet was ambushed by the malevolent forces of the Warp, crippling or killing many of their Navigators and Astropaths  in the initial onslaught. The Blood Angels now faced the fury of Chaos for the first time. Kyriss sent an image of himself to Sanguinius, declaring his lordship over the system in the name of Slaanesh and taunting the Primarch into taking it back from him. Though they had never faced such a foe, the Blood Angels prepared their counterattack, sure that they would prevail. Rising to the challenge of the vile Greater Daemon, the Blood Angels Legion attacked the daemon host of Kyriss, launching a series of attacks across the seat of the daemon's power, the world of Signus Prime. During the epic battle, Sanguinius came face-to-face with a new nightmare known as Ka'bandha, a Greater Daemon of Khorne. During the battle that ensued, Sanguinius was sorely wounded and temporarily incapacitated. He witnessed Ka'bandha slaughter 500 of his sons with huge swathes of his mighty axe. The psychic backlash of the deaths of so many of his sons blasted Sanguinius into unconsciousness. With the fall of their Primarch and the slaughtering of their brethren, the Blood Angels Legion was consumed by a black rage that drove them into a berserker's fury as they charged into the daemonic horde and in their madness they smashed the horde of daemons asunder. Yet the brutal violence of the daemon Ka'Bahnda had unleashed something dark within the psyche of the Space Marines, a thirst for blood that would not be slaked until every taint of Chaos had been erased from the planet. Even the mighty Kyriss was banished back to the Immaterium. Only when the planet was cleansed did the rage of the Blood Angels finally subside. Though Signus had been freed from its thrall to the Forces of Chaos, the cost of victory was far higher than any could have wished. The berserker rage the Blood Angels had experienced had left a brooding shadow on their souls that would manifest in the centuries to come as the great curse (later known as the Black Rage) that would afflict the Blood Angels and their later Successors.

Landing on Terra
The Battle of Terra began with an orbital bombardment by the Warmaster Horus' fleet as the prelude to invasion. Although the Loyalist fleets and defenders fought back and the massive orbital defences on Luna reaped more than a quarter of the starships in the Traitor fleet they, like the Loyalist soldiers on the surface, were too few to face the combined forces of so many Traitor Legions, and were mowed down without mercy. After days of bombardment, the Chaos Space Marines landed on the surface of Terra in Drop Pods and advanced on the two spaceports nearest the location of the Imperial Palace to secure them in preparation for the main landings of the Traitor forces. Elements from five of the Traitor Legions participated in the battle, aided by the Traitor forces already on the surface. Despite the brave efforts of the Loyalists, the Eternity Wall and the Lion's Gate Spaceports fell within hours to the Forces of Chaos. Dark Chaos Cultists made their invocations, calling down the Greater Daemons of Chaos from the Warp directly onto Terran soil. With the spaceports secured, Horus' remaining troops of the Traitor Legions and their Traitor Imperial Army and Dark Mechanicus support forces landed en masse, and the hulking transports carried thousands of troops each. They also landed the terrible Traitor Titans that served the Warmaster's cause and had been infected with the daemonic power of Chaos. The transports' immense size made them prime targets for Terra's defence lasers. Although many of the Traitor landing craft were destroyed in-atmosphere, many more made it to the surface, disgorging yet more soldiers, main battle tanks and Traitor Titans to add to the besiegers' strength. They met stiff resistance from the Loyalists as the Imperial defenders knew that the survival of the Mankind's homeworld, their Emperor, and the future of the entire human race rested on their shoulders.

Siege of the Imperial Palace


The Chaotic besiegers forced the Imperial defenders back to the walls of the Imperial Palace, where thousands died slowing the assault. The Primarch Angron of the World Eaters Legion, now a Daemon Prince of the Blood God Khorne, came forth before the walls of the Palace and demanded the Loyalists' surrender, saying that they were cut off, outnumbered, and defended a ruler unworthy of their loyalty. Many would have surrendered to Angron after seeing the sheer power of the Forces of Chaos that stood arrayed before them had it not been for the Primarch Sanguinius, the winged and seemingly angelic leader of the Blood Angels Legion. The two Primarchs, once brothers, gazed at each other, perhaps communicating telepathically. Eventually Angron withdrew from before the gates of the Imperial Palace, telling his forces, not without some relish at the prospect of slaughter, that there would be no surrender.

The siege of the Imperial Palace then began in earnest. Three times the Forces of Chaos scaled the walls, and three times were hurled back by Sanguinius and his Blood Angels. Outside the Palace walls, Space Marine and Imperial Army forces led by Jaghatai Khan, the Primarch of the White Scars Legion, unsuccessfully tried to draw the bulk of the besiegers' army away from the Palace. Soon the outnumbered defenders were pushed back into the maze of corridors and bulwarks within the Palace walls. Frustrated with his army's slow progress, Horus ordered the Legio Mortis (Death's Head Legion), a Traitor Titan Legion, to demolish entire sections of the wall. Despite grievous losses, the Titans, led by the infamous Imperator-class Battle Titan Dies Irae, gouged open breaches in the Imperial Palace's defences, which the Traitors then flooded through.

Facing a breach and potential collapse of the Imperial defences, Jaghatai Khan decided on a change of plan. Rather than assaulting the almost-invincible flanks of the Chaotic army, Khan redirected his highly mobile White Scars Space Marines and the surviving Loyalist Tank Divisions of the Imperial Army to Lion's Gate Spaceport. At dawn Jaghatai's lightning raid caught the Traitor garrison at the spaceport completely by surprise, and reclaimed the spaceport for the Imperium. The Khan ordered his troops to reactivate the spaceport's defence lasers to prevent the Traitor fleet from bringing down any more troops and equipment and form a defensive perimeter to hold their newly reconquered territory. Khan's troops repelled several frenzied counter-attacks from the Traitors, and began firing on Horus' unprotected drop ships. The Khan's plan worked perfectly: the flow of the Traitors' men and machines to the Imperial Palace had been cut in half at a single stroke. Inspired by this success, the Loyalists also tried to seize back the Eternity Wall Spaceport, but were driven back by the Chaos forces without difficulty, as they had reinforced their garrison following the loss of the Lion's Gate.

Inside the Palace, the defenders had been forced back to the Eternity Gate, the sole point of entry into the inner sanctum of the Imperial Palace. The Astartes of the Blood Angels and Imperial Fists tried to hold back the attacking Chaotic troops, while the remaining Loyalists made it through the Gate. Soon the mighty Bloodthirster Greater Daemon Ka'bandha came forth and bellowed out a challenge to Sanguinius in the name of his master Khorne. The daemon hurled itself at the Angel of Baal, barely allowing him time to parry the daemon's strikes. The two took to the air, trading blows and battle cries high above the heads of the two forces. Already fatigued from the long siege, Sanguinius was cast down by the daemon, pulverising the ferrocrete below upon impact. The Loyalist forces seemed to collectively groan at the fall of their great champion.

Yet the Blood Angels' Primarch was not beaten, only stunned by the force of the impact. Sanguinius cleared his head, forced himself back to his feet, and once again took to the sky. The Angel seized the gloating daemon, holding it by the right ankle and arm. The Primarch hefted the creature high and broke its back over his knee, before hurling the daemon's carcass back at the besiegers, who howled in despair as the last Loyalists fell back and made it into the Imperial Palace's inner sanctum before the great portal of the Eternity Gate was shut tight behind them. Of course, as a daemon, Ka'bandah could not truly be slain, only banished to the Warp for a 1000 standard years, but the Bloodthirster's spirit was sent howling back into the Immaterium to meet the displeasure of his master the Blood God.

The Eternity Gate was closed.

Inquisition
The Warp Gate the Emperor had constructed deep beneath the Imperial Palace and the short section of Webway passage beyond required constant maintenance lest they fall into ruin. At first this demanded only a small portion of the Emperor's psychic might and so He was able to command His armies and do all that was expected of Him as Emperor. But the hideous monstrosities that ruled the Warp -- the self-proclaimed Gods of Chaos -- had ever been his foes, and now conspired to subvert the Emperor's goals as they had since the day He had launched the Great Crusade. To this end they had tempted the naive Magnus the Red to warn Him of the very plot they had initiated, the betrayal of the Imperium by Horus. Magnus sent his warning by means of powerful psychic sorcery and this broadcast had wreaked havoc upon the protective psychic shielding surrounding the Emperor's fragile Webway construct. The spell of Magnus not only allowed the foul denizens of the Warp entry to the section of the Webway the Emperor's secret army of Adepts and Tech-priests had by then conquered, it destroyed the delicate controls the Emperor had set in place. Now the Warp Gate he had constructed required virtually all of His psychic power and mental concentration lest it rip open a permanent doorway between Terra and the Warp, flooding the homeworld of Mankind with the daemonic legions of the Ruinous Powers.

The Emperor told Malcador that he had to take the Emperor's place on the psychic amplifier known as the Golden Throne, which provided the psychic sheath needed to protect the new, human-built sections of the Webway intended to be the Emperor's final gift to humanity before the Horus Heresy had begun. The Emperor's original choice of His replacement on the artefact had been the Primarch Magnus the Red, but since Magnus and his Thousand Sons Space Marine Legion had sided with Horus and the Chaos God Tzeentch, Malcador was now His chosen successor and the only remaining human psyker with enough strength to carry out the duty.

In the days before the final confrontation between the Emperor and Horus aboard his Battle-Barge the Vengeful Spirit during the Battle of Terra, the Emperor ordered Malcador to summon twelve "..men of character, skill and determination" who would be tested and trained to become the elite group of investigators intended to root out treachery across the Imperium in the centuries to come to prevent any event like the Horus Heresy from occurring again. The Emperor also told Malcador to prepare himself for the dreadful sacrifice that he would be called upon to make.

Malcador the Hero
The Terran Warp Gate would remain closed to the daemons for as long as the Emperor was able to power it from His throne atop the golden portal. Only the mightiest of human psykers had power enough to do this and even then most would be exhausted and fail in a short time. Only the Emperor Himself had the might to keep the gate closed permanently and for Him the effort grew harder as the daemonic forces gathered about Him. For as long as the daemon horde threatened to breach the portal, the Golden Throne would be His prison. As Horus' forces began their final assault on the Sol System and the Battle of Terra began seven standard years after the Traitors had first turned upon the servants of the Emperor at Istvaan III, the Sigillite returned from his mission to recruit the foundation of the Inquisition. Only through the most artful of psychic subterfuge were Malcador and his new recruits able to pass unscathed through the battlelines and come unharmed and unseen before the Emperor within the inner sanctum of the Imperial Palace.

Malcador had finally received the call and was now prepared to perform his final duty to the man he had followed for the greater part of his life. Once within the depths of the Imperial Palace, the Emperor asked if Malcador was prepared to take his place upon the Golden Throne. Ever loyal, the Sigillite was more than willing to sacrifice himself for his Emperor. But before he ascended to take his place upon the Throne, the Sigillite had one last duty to perform. He was accompanied by a group of twelve hooded attendants. In stern silence the Emperor surveyed the robed figures that Malcador had brought before him, and he saw that his faithful servant had done well. Of the twelve, four were mortal lords and administrators of the Imperium possessed of an inquisitive nature and unyielding strength of mind. The other eight were Space Marines whose abilities were as peerless as their dedication to the Emperor. Some hailed from Legions that had abandoned the Emperor's light in favour of Horus' dark promises, but these Battle-Brothers had never lost their loyalty and had fought the Heresy from within. Fulsome in his approval of the selection, Malcador the Sigillite ascended to the Golden Throne, replacing the Emperor who now stood before the edifice with his loyal captains Rogal Dorn and Sanguinius.

Malcador could not speak, such was the concentration he had to bring to bear in order to control the tempestous forces at his call. The Emperor directed the attention of the two mighty Primarchs, "Behold the greatest sacrifice of our age! Malcador the Sigillite is no more. Henceforth he shall always and only ever be Malcador the Hero!" At this the three figures retired from the Imperial Palace's vault and made ready to teleport onto the Battle-Barge of Horus. The task of keeping the daemons out of the Imperial Palace was daunting for the Emperor, the mighty reincarnation of a thousand human psykers with millennia of experience to call upon. Though he was a powerful psyker in his own right, Malcador was still but a mere human, his mental powers nothing compared to that of the Emperor, and this task proved overwhelming, consuming him body and soul in a matter of hours.

Endgame
The siege of Terra following the initial assault on the Imperial Palace lasted for 55 days. Both sides knew that the defeat of the Imperium of Man was near after the defence of the Eternity Gate. Sensing this, and knowing that he must complete the siege before the arrival of Loyalist reinforcements from the other Space Marine Legions that were already on their way, Horus prepared to teleport to the surface from his flagship, the Vengeful Spirit, to lead his forces in person. Before this could happen, the Word Bearers' First Chaplain Erebus broke the news to Horus: their daemonic allies in the Warp had informed them that the Dark Angels and Space Wolves Legions were nearing Terra; and the Ultramarines were only a short distance behind.

At that moment, Horus despaired; his gamble had failed, weeks of further conflict would be needed to break the defenders and the Emperor's reinforcements would arrive in mere hours. What happened next is disputed in the Imperial historiography of the Heresy; some believe Horus disabled his Void Shields as he experienced one last moment of regret for his betrayal of his father and his turn to Chaos, while others believe it was a personal challenge to the Emperor. Nevertheless, Horus lowered the Void Shields of his flagship, the enormous Battle Barge Vengeful Spirit. The lowering of the vessel's shields was detected by the Loyalist vessels in orbit and the information was relayed to the Imperial Palace.

The Emperor of Mankind rose to the challenge, leading members of His elite personal guard, the Legio Custodes, the Primarchs Sanguinius and Rogal Dorn, and several companies of Imperial Fists and Blood Angels Veteran Space Marines in the assault and teleported aboard the Vengeful Spirit. Horus used his Chaotic powers to scatter the Emperor's force throughout the massive warship when they teleported up through the Warp. Each fought a series of battles against the elite Forces of Chaos aboard the corrupted starship, attempting to link up with their comrades and confront Horus. It was Sanguinius who reached his brother Horus first. The Warmaster attempted to turn the Blood Angels' Primarch, his oldest and closest friend among the other Primarchs, to Chaos one last time. When Sanguinius refused to be corrupted, Horus attacked. Wounded from his many battles on Terra and the terrible battle with the daemon Ka'bandah, Sanguinius proved to be no match for Horus, now at the peak of his daemonic power after his long alliance with the Ruinous Powers. Horus strangled the Angel of Baal with ease. An alternate version of this event sometimes recorded in the Imperial records has Sanguinius cutting a small hole in Horus' Terminator Armour before he died, as this hole aided in the Emperor's final defeat of Horus.

When the Emperor finally entered the throne room of the Vengeful Spirit, he saw the winged corpse of the angelic Sanguinius lying at Horus' feet. Horus called the Emperor foolish for refusing the power that the Chaos Gods offered to Men, and timid for not taming them to His will if He was truly the Master of Mankind as He claimed. Horus proclaimed that if the Emperor would kneel before him, then he would spare his life. But the Emperor, tens of millennia older than his misguided and once beloved son, knew well the ancient trap that had snared Horus. The Emperor told the corrupted Primarch that he was the deluded slave of Chaos, not its master, for no mortal could ever truly claim to be more than simply a pawn of the Ruinous Powers. Snarling, Horus hurled bolts of daemonic lightning at the Emperor, but the Emperor nullified them with His own immense psychic abilities. The die was cast. Each god-like being knew that the fate of humanity now hung in the balance.

The Emperor and Horus engaged one another in the throne room of the massive Battleship, a battle that was both physical and psychic in nature. Though the Emperor's psychic gifts and martial skills were unequaled, He found Himself unwilling to summon His full strength against His beloved son. The Emperor suffered grievous wounds at Horus' hands, and after a score of thrusts, parries and counter-thrusts between the Emperor's Runesword and his own Lightning Claw, Horus sliced open the Emperor's chest armour, then opened his jugular and severed the tendons in his right wrist, disarming the Emperor. A psychic blast seared the flesh from the Emperor's face, destroying one of the Master of Mankind's eyes. After tearing the Emperor's right arm from its socket, Horus raised his father's broken body high over his head, and broke his back over his knee.

At that moment, a lone Loyalist warrior entered the bridge, just as the Emperor fell. Horus, gloating in victory, showed the Imperial warrior the Emperor's broken form and laughed, taunting the Loyalist's uselessness and defeat. The valiant Imperial warrior, far from being daunted, roared in defiance and interposed himself between the Emperor and the Warmaster, heroically holding the line against the Chaos-corrupted Primarch. He was flayed alive for his defiance by a glancing psychic blast from Horus.

The exact identity of the brave warrior is a source of much debate amongst Imperial historians, for today only the Emperor knows the truth. The Adeptus Ministorum tell the story that it was a lone Imperial Army trooper named Ollanius Pius who held the line, going as far as to canonise the man as the Guardian Saint of the Imperial Guard in the 32nd Millennium, yet many doubt that mere humans would have accompanied the Emperor and His Primarchs to confront Horus. The Imperial Fists maintain it was one of their Terminators who challenged Horus, however some see this as a desperate attempt to lessen their collective guilt stemming from the fact that Rogal Dorn never could participate in the fighting. Most savants now agree that it was probably one of the Adeptus Custodes who interceded, yet the other stories continue to be told and retold over the whole of the Imperium of Man.

The casual brutality of the Warmaster's act galvanised the Emperor as He realised what awaited Mankind under the rule of Horus and the Chaos Gods. Realising at last that His favoured son was truly lost to the corruption of Chaos, the Emperor finally gathered His full and awesome psychic power in the Immaterium and unleashed a lance of pure Warp energy that pierced the gloating Horus' psychic defences and ripped his body apart. In some versions of the tale, this blast was only able to pierce Horus' body through the hole that had been made by Sanguinius before his death. Just before Horus died, he looked his father in the eye, shedding a single tear, begging his father to forgive him for his betrayal. The Emperor saw regret in his fallen son's eyes. The Emperor also knew that the Ruinous Powers could attempt to possess Horus again, and that He would not be there to stop his son again if they did. Driving all of the near-infinite reserves of compassion from His mind for the sake of the humanity He had served and loved all the years of His long life, the Emperor destroyed Horus utterly, his essence burned from existence in both the physical world and the Immaterium so that the Ruinous Powers could not resurrect Horus as a Daemon Prince through their claim on his soul.

The destruction of Horus' soul sent a psychic shockwave surging across the Solar System, casting the daemons of Chaos back into the Warp, and spreading mass panic among the Traitor Legions and other Traitor forces on the surface of Terra in seconds as the Chaos Gods found their powers disrupted temporarily by the death of their favoured mortal vessel. It became clear to the Forces of Chaos that their leader had been defeated. A terrible, berserk fury later known as the Black Rage had encompassed the Blood Angels at the moment of their Primarch's death, and they were surging forth to scatter the attackers. Retreat turned to rout, and rout soon turned to bloodbath; thousands upon thousands of Chaos Space Marines and Chaos Titans fell attempting to flee. The ground before the Sanctum Imperialis ran red with the blood of Traitors and Heretics.

After Death
The tragic tale of Horus does not end with his death aboard the Vengeful Spirit. His body was enshrined on the Daemon World the Sons of Horus claimed for their own within the Eye of Terror after they fled Terra and renamed themselves once more as the Black Legion. Horus' corpse resided there for several hundred years, before the body was stolen by the corrupted Apothecary Fabius Bile and the Emperor's Children Traitor Legion, in an attempt to clone the body of the Warmaster to bring Horus back to life so that he might lead the Traitor Legions once more in an attempt to conquer the Imperium. Abaddon led an assault on the Emperor's Children's fortress, and believing that the continued worship of his Legion's dead Primarch had trapped the Traitor Legions and led them to the brink of destruction, Abaddon utterly destroyed Horus' corpse and claimed the Warmaster's Power Claw, the Talon of Horus, as his own, as well as the title of Warmaster of the Forces of Chaos.

Wargear

 * Serpent's Scales - Horus' unique suit of Terminator Armour, known as the Serpent's Scales, was one of the first prototypes of its kind, fashioned by Master Adept Urtzi Malevolus and continuously improved by the hand of the traitorous Fabricator-General of Mars, Kelbor-Hal, and some of the greatest Artificers of the Imperium. It was proof against attacks of both brute and esoteric origin.
 * Worldbreaker - Worldbreaker was a Power Maul the size of a mortal man marked out with the Imperial Aquila at the base of its grip. As well as being a weapon capable of shattering armoured Ceramite, it was also a signifier of Horus' rank as the Warmaster of the Imperium. It is said to have been created by the hand of the Emperor Himself and presented as a gift to his favoured son on the day Horus was raised to become the Warmaster and the Emperor announced that he was returning to Terra.
 * Warmaster's Talon - The Warmaster's Talon is a unique Lightning Claw which incorporated a baroquely styled twin-Bolter that was a precursor of a modern Storm Bolter. The Talon had long been Horus' favoured weapon. Some apocryphal sources claim it was an antediluvian relic that was found deep within the planet Cthonia, and was a product of Mankind's Dark Age of Technology.