Horus Heresy

"Sometimes the only victory possible is to keep your opponent from winning."

- The Emperor of Mankind before the Battle of Terra

The Horus Heresy was a galaxy-spanning civil war that consumed the worlds of Mankind for 7 Terran years. Its outbreak marked the end of the Emperor of Mankind's Great Crusade to reunite the scattered colony worlds of humanity under a single government and the beginning of the current Age of the Imperium. The Horus Heresy is in many ways the founding event of the Imperium of Man as it now exists. The conflict was fought across the Milky Way Galaxy in the early centuries of the 31st Millennium and resulted in more than 2.3 trillion dead, 4.6 trillion if one includes the planetary populations purged by the Imperium after the Heresy due to the taint of Chaotic corruption. The Heresy concluded with the death of the traitorous Warmaster Horus, the internment of the Emperor in the Golden Throne and the creation of the Chaos Space Marine Traitor Legions. The Heresy was directly responsible for the birth of the present-day structure of the Space Marine Chapters following the Second Founding and the Reformation of the Imperium by the Ultramarines' Primarch Roboute Guilliman.

Wars of Unification
During the turbulent era known as the Age of Strife, the Sol System and the nearby star systems that had been colonised by humanity during the Dark Age of Technology were effectively cut off from interstellar travel or communication with each other due to the massive Warp Storms that swept the galaxy as the Immaterium was roiled by the millennia-long gestation of the Chaos God Slaanesh and the turbulence that marked the decay of the Eldar empire before the Fall. During this dark time, Terra sometimes held sway over the Sol System, while at other times the rulers of Mars or Lunaweredominant. The different worlds found themselves constantly at war. During this long, 5,000-year-period of anarchy, fear and violence, Old Earth's once unified planetary government had completely broken down and been divided into dozens of warring states of so-called techno-barbarians. Continuous warfare raged across the surface of Terra for 2500 years, beginning in the late 27th Millennium. Little remained of the once sophisticated civilisation of Old Earth's glorious past as the center of a growing human interstellar civilisation marked by advanced science, high culture and wondrous technologies. Techno-barbarian warlords and their warrior hordes continuously fought over the planet, which had become little more than a massive battleground for their wars of attrition. They made use of chemical, biological and even thermonuclear weapons of mass destruction, and slowly transformed the cradle of Mankind into a battered, post-apocalyptic wasteland across most of its scarred surface. This was a dark period for the people of Old Earth when they were dominated by brutal rulers and despotic tyrants. It was against this backdrop of oppression, violence and casual brutality that the Emperor of Mankind first revealed himself to the people of Terra. In secret, he had been planning for this moment in history for millennia, ever since the Age of Strife had fractured what remained of the ancient human federation. With his massive army of genetically-enhanced warriors who comprised the first units of the Imperial Army and would serve as the prototypes for the later development of the Space Marines, the Emperor began his conquest of Terra with the intent to reunite the warring nations into a unified planetary government and then use Terra as the springboard from which to begin his reconquest of the galaxy under the aegis of an Imperium of Man dedicated to the Imperial Truth of progress and reason. Brutal rulers such as the warlord Kalagann of Ursh, Cardinal Tang of the Yndonesic Bloc, and the most infamous of all, the half-mad, half-genius Nathaniel Dume, the Tyrant of the Pan-Pacific Empire, would all fall by the wayside. Once unleashed, the armies of the Emperor swept all before them like wheat before the harvester's scythe until all the techno-barbarian warlords had either been conquered outright or had agreed through diplomacy to become subservient to the Emperor's will.

During this time the Emperor created a number of military organisations, such as the Imperial Army, which would become the nucleus of the armed forces that would support his Legiones Astartes and his reconquest of the human-settled galaxy. Amongst the early Imperial Army units that saw action during the Unification Wars were such venerable regiments as the Geno Five-Two Chiliad, which would become one of the oldest and most respected regiments within the Imperial Army. Like many of these early regiments, the geno-soldiers were created through the use of some of the Emperor's sophisticated genetic engineering techniques, developed in his laboratories beneath the Himalayan Mountains and on Luna. The most elite of this first generation of genetically-engineered supersoldiers were known as the Thunder Warriors, men who were physically stronger and more formidable in combat than even the later Space Marines, though they were engineered to be vicious killers and lacked many of the more noble aspects of the Astartes as well as their tremendously long lifespans. The Thunder Warriors were engineered to be the means to an end and were never intended to be integrated into the Emperor's new realm after Unity had been achieved. Through genetic engineering and selection the standard geno-soldiers displayed many of the characteristics of the perfect human warrior -- they were physically more resilient, stronger and capable of taking more damage than any of their unaltered techno-barbarian foes. These early genetically-engineered Imperial Army regiments would go on to continue serving the Emperor after the conclusion of the Unification Wars. These regiments would eventually be referred to as the Old Hundred, and would form the core of the initial military force that embarked upon the Emperor's galaxy-spanning Great Crusade in the early 31st Millennium beside his newly created Space Marine Legions.

When the Unification Wars were complete at the end of the 30th Millennium, the Emperor forged a new unified planetary government under his leadership. He next journeyed to Mars and met with the Adepts of the Cult Mechanicus. In return for the use of the Mechanicus' vast manufactorums, the use of its Titan Legions and the orbital shipyards to construct the weaponry and starships he would need for the Great Crusade to reunite Mankind under the banner of the Imperial Truth, the Emperor agreed to grant the newborn Adeptus Mechanicus complete autonomy on Mars and its other Forge Worlds as well as an exemption to the atheism required by the Imperial Truth. This agreement, known as the Treaty of Mars (the Treaty of Olympus to the Mechanicus), marks the true foundation of the Imperium of Man in the alliance between Terra and Mars.

Council of War
Following the successful conclusion of the Unification Wars in circa 800.M30, the Emperor convened the ruling body known as the War Council to manage the execution of the Great Crusade to reunite the entire human-settled galaxy under a single government. The War Council effectively became the true ruling body of the Imperium during the early and middle years of the Great Crusade. The Emperor Himself sat at the head of the Council; at his left hand was Malcador the Sigillite, perhaps the Emperor's greatest ally during the Wars of Unity and a psyker whose powers were matched only by those of the Emperor. The rest of the Council was composed of talented administrators drawn from the great ruling aristocratic dynasties of Terra and the Segmentum Solar, and when the Emperor forged his alliance with the Mechanicum of Mars in the Treaty of Mars, the Fabricator-General of the Mechanicus also claimed his seat. The War Council was also attended by the Paternova of the Navigator Houses. As the Emperor left the homeworld of Mankind to lead the Great Crusade into the stars, he left the legendary Malcador to act as the Regent of Terra in his stead.

The Great Crusade
When the great Warp Storms that had cut off Terra since the end of the Dark Age of Technology subsided, and the Age of Strife came to an end at the dawn of the 31st Millennium, the Emperor of Mankind deemed it time to begin his Great Crusade, a massive galactic campaign by which he and his armies would free all human-settled colony worlds from alien oppression or primitive ignorance and reunite the human race across the galaxy under the single banner of the new Imperium of Man. To execute this plan, the Emperor created the Primarchs, his god-like, genetically-engineered superhuman offspring. The Primarchs were still in their infancy, growing to hyper-accelerated maturity in their special gestation tanks, when they were snatched away from the genetic laboratory deep beneath the Himalayan Mountains on Terra where they had been created and gestated by the Emperor using his own DNA. The cause of this disappearance was the Chaos Gods, who were fearful that with the Primarchs the Emperor would be able to impose Order across the galaxy and weaken their own firm grip on the collective unconsciousness of the minds of men. Uniting their powers, the Dark Gods opened a portal from the Warp and broke into the laboratory through the psychic wards the Emperor had erected. Unable to destroy the Primarchs outright because of the wards the Emperor had laid upon them, the Chaos Gods instead chose to scatter them across the galaxy through the Warp. The superhuman infants eventually came to rest on diverse, human-inhabited worlds. It was at this time that the Ruinous Powers first touched the souls of those Primarchs who would eventually turn to the worship of Chaos, infecting them with a shard of corruption. Certainly the fact that each of the Primarchs was ultimately cast ashore on an inhabited human world was no accident; the Chaos Gods may have hoped that by having the Primarchs raised among men without the guidance of the Emperor they would display more of the human weaknesses that would make them easier to corrupt when the time came.

During the Great Crusade, the Emperor encountered each of the Primarchs on their scattered homeworlds in turn. To fill the gap in his military plans for the Great Crusade wrought by the Primarchs absence, the Emperor had instead Founded 20 Space Marine Legions. The Emperor used the Primarchs' genetic material still in his possession to craft the genetically-enhanced warriors of each Legion. After the Emperor rediscovered his sons he deemed it fitting that each Primarch should lead their genetic offspring as the commander of the Legion whose Astartes bore their DNA. However, this would prove a critical mistake, as the Space Marine gene-seed creation process made the Emperor, the Primarchs and the Space Marines analogous to Grandfather, Fathers and Sons, respectively, in their genetic inheritance and superhuman abilities. In time, many of the Space Marines in the Legions, especially those recruited from their Primarchs' homeworld rather than from TerrabeforethePrimarchs' rediscovery, would come to venerate and feel more loyalty for their Primarch than the Emperor of Mankind.

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 cam from mine own mouth. But words of caution I have for you for your brother Primarchs are strong of will, of though 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, at the Triumph of Ullanor

The Ullanor Crusade was a vast Imperial assault on the Ork empire of the Overlord Urrlak Urrukduring the late 30th or early 31st Millennium. This massive undertaking would mark the culmination of the Great Crusade's expansion. Ullanor, the capital world of this empire, and the site of the final assault, lay in the 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 Orks ever defeated by the military forces of the Imperium of Man before the Third War for Armageddon began during the late 41st Millennium.

With the great victory sealed in blood and iron, the call to a Triumph was sounded, to recognise this highpoint of the Great Crusade and to honour all the warriors of the Ullanor Crusade, mortals and Astartes alike, for their extraordinary valour and service to humanity's cause. By the Emperor’s command, Ullanor was remade as a trophy world, designated Mundus Tropaeum on all galactic maps and records of tithe. It would be a site of glory and spectacle to cement not only this single conquest over the forces ranged against Mankind, but a greater symbol of the Great Crusade itself. For two hundred Terran years the Emperor’s mighty endeavour had moved across the face of the galaxy to bring unity and illumination to the lost daughter-worlds of Old Earth. It had pushed back the night, reforged old links between civilisations, battled alien threats -- and with regret, it had often punished those who refused to return to the Imperial fold. A change was coming, though, a change that found its fulcrum on Ullanor. None who walked upon that world knew that the echo of that Triumph would sound for decades, for centuries, for millennia. The glory of this triumphant spectacle as so many of the Imperium's scattered military forces gathered in one place for the first time in centuries was to remain in the mind of every Astartes as the high-point of the great endeavour that they were engaged upon. It would prove to be a bright memory to recall in the dark days of the Horus Heresy after Astartes had turned against Astartes and Primarch against Primarch.

To prepare the world for the Imperial Triumph, geoformer platoons from the Adeptus Mechanicus brought world engines and mobile stone-burners to cut a massive swath across the broken landscape left in the battle’s wake. Orkish dead were buried by the millions with their savage ruins, interred beneath transplanted rocks and the heads of crushed mountains. The Mechanicus eradicated every last remaining trace of the enemy and paved over them with a giant boulevard, a parade stage as wide as the footprint of some entire Imperial cities. They built a highway and allowed only one structure to stand besides the great platform -- an ornamental pavilion of black marble and heavy granite that had been built piecemeal on Terra and then shipped across the void by special envoy. Marker posts decorated with the skulls of Ork commanders paced out the length of the road, and behind them great bowls of smokeless Promethium burned brightly, endlessly lighting the highway with their blue-white fire.

When the Mechanicus had finished their work, the honoured came to pay homage to the battle won, the Great Crusade’s ideal of human unity and the Emperor who was father to all Mankind. The Imperial Army and the Titan Legions bracketed the gathering. Human troops were ranked in uncountable numbers, their host so wide they became a sea of battle armour and dress uniforms. Every common man and woman who stood on Ullanor’s soil that day had been selected for their valour and conduct, and until the day they died each would have the singular honour of wearing the onyx-and-gold Ullanor Triumph Bar upon their uniforms. The award was forged from Bolter shells recovered from the field and melted down. Ranged around them, the great war machines of the Collegia Titanica towered towards a sky cut to ribbons by the contrails of a thousand aerospace fighters; and above those, high over the thin white cirrus clouds of Ullanor’s day, Imperial warships moved as slow as they dared through the upper atmosphere, washes of interface heat rolling off their Void Shields as they showed their flanks in a gesture of renewed fealty.

A full fourteen of the Space Marine Legions stood represented at the Ullanor Triumph, and with them came nine beings of superhuman power and majesty. Nine gods and angels made flesh, the Primarchs of the greatest armies ever created by human hands. Mortarion, the reaper of men and master of the Death Guard, cowled and lethal in aspect, matched by the warrior-guardians of his Deathshroud honour guard. The Phoenician, Fulgrim, resplendent in his finery and handsome in aspect, lit by the reflection of gold and platinum. Magnus the Red, the Crimson King, the lord of the unknown, his soul as much a mystery to the common world as the workings of the Warp and the ghosts within it. Lorgar Aurelian, the quiet and brooding zealot who burned with such intensity and buried it all deep in his heart, saying little and standing watchful. His polar opposite was Angron, the gladiator-lord and son of grief, never able to settle or moderate his seething, endless fury, always on the verge of outburst and violence. Rogal Dorn, the stalwart man of stone, the Imperial Fist with his unswerving manner and unbreakable focus, the one who would always obey, would always be ready for duty. Jaghatai Khan, his fur-trimmed robes and ornate armour detailed with a thousand narratives of the White Scars Legion, his every step across the land a challenge to the galaxy. Then Sanguinius of the Blood Angels, flanked by the gold-armoured honour detail of the Sanguinary Guard, his mighty wings folded back across his battle-plate, his face turned to the sky to welcome the impossible, majestic sight before him. Then, finally, came Horus of the Luna Wolves, the Hero of Ullanor, liberator and first among equals. Horus, who was to be given the new honour of an Imperial title above and beyond any that had been bestowed before; a title, it could be said, that would forever carry the echo of his name. After declaring Horus as Imperial Warmaster and the new supreme commander of the Great Crusade, the Emperor to the shock of those assembled announced his own intention to return to Terra to pursue a secret project intended to benefit all Mankind.

Seeds of Heresy Sown
After more than 200 years of hard conflict in the early 31st Millennium, over two million human-settled worlds across the Milky Way Galaxy had been reclaimed by the Emperor in the name of the Imperium of Man. Beside him had stood the Primarch Horus, who had fought alongside the Emperor for the early part of the Great Crusade as his only rediscovered son and primary military commander, glorying in the attentions of his father. The long wars had forged a strong bond between them, and they were truly father and son. But now the Emperor had to consolidate his newborn Imperium, and undertake the next phase of his Grand Plan. He intended to create a Webway through the Immaterium much like that used by the Eldar that would be open to humanity by wielding the psychic augmentation technology of the artefact from the Dark Age of Technology known as the Golden Throne. This work required his continued presence on Terra, and so after Horus' magnificent victory in the Ullanor Crusade, against the largest horde of Orks ever encountered in the galaxy at that time, the Emperor departed and left Horus in charge of the Great Crusade with the exclusive title of "Warmaster." As Warmaster, Horus was the Commander-in-Chief of all of the Imperium of Man's armies, charged with leading the other Primarchs and their Legions through the remainder of the Great Crusade. At this announcement there was much shock and outrage. Many of the other Primarchs did not understand why the Emperor was leaving them to fight the enemies of Mankind alone and, worse still, why Horus should be raised as the first among equals. Rogal Dorn, Sanguinius and Fulgrim were pleased for their brother, the new Warmaster, while others -- such as Angron, Roboute Guilliman, Lion El'Jonson and Perturabo -- all reacted with varying degrees of disapproval. The situation only grew worse when the Emperor announced that he would be creating a civilian administrative bureaucracy known as the Council of Terra comprised of Imperial bureaucrats and nobles to carry out the day-to-day governmental affairs of the Imperium, replacing the direct rule of the Emperor while he was engaged in his secret Imperial Webway Project. The Council of Terra would implement and administer the new galaxy-wide tax of resources and manpower called the Imperial Tithe and other matters of day-to-day law in the Imperium of Man while the Emperor focused on bringing his Webway Project to fruition using the technology of the Golden Throne. The Primarchs would be relegated to a primarily military role as the Imperium's most preeminent commanders. Many of the Primarchs, including Horus, were deeply disturbed that their father would make them subject to normal men and women who had never shed blood in the establishment or expansion of the Imperium and that he would remove them from the political positions of rule to which they believed they were entitled. The Emperor had sought to create a civilian bureaucracy for the Imperium precisely because he wanted regular human beings to learn to govern themselves once more and not become beholden to a permanent, genetically-enhanced ruling class. The Chaos Gods would ultimately use the Primarchs' resentment as one of the all-too-human weaknesses they could exploit to corrupt half of their number.

The Lords of the Imperium
After the decisive victory during the Ullanor Crusade, when Mankind's re-ascension to predominance in the galaxy was no longer in doubt, the Emperor bestowed upon the Primarch Horus Lupercal the title of Imperial Warmaster and ceded to him control of all the Imperium's military forces in the Emperor's stead. The other Primarchs were then instructed to follow Horus and obey him and to complete the Great Crusade under his direction. There was, it is said, some disquiet among the Primarchs that the Emperor had decided to no longer fight alongside them, but the Emperor was as adamant as he was close-mouthed as to what he would do on his return to Terra. The Emperor then departed for the homeworld of Mankind and the dungeons deep beneath his great Imperial Palace to begin His great work on te creation of a human extension into the Eldar Webway under a veil of secrecy previously unknown in the Imperium. He drew to him certain advisors and retired to the private vaults of his city-fortress.

Upon his return to Terra, the Emperor called to his side Malcador and the Fabricator-General. He issued them with new commands. No longer were they to support the military campaigns, as these were now safely in the hands of his sons the Primarchs and the newly appointed Warmaster Horus. The Emperor needed time and all of his focus to be directed at his next great project, which would tie the newborn Imperium together and unite it as no other human polity had ever been united. To this end, the Emperor convened the first Council of Terra. Unlike the War Council, of which Horus was now the leader, the Council of Terra would attend to matters of state and the establishment and maintenance of Imperial Law across the myriad worlds of the Imperium. In particular, the Council of Terra was to administrate the establishment of the Imperial Tithe of troops and resources from all the worlds of the Imperium that were required to support the Great Crusade. In effect, under the auspices of the new Council of Terra would fall the entirety of the civil government of the Imperium. Malcador, the Emperor's most trusted advisor, was named as the First Lord of the Council and would lead it in the Emperor's absence. The Fabricator-General of the Mechanicum of Mars, Kelbor-Hal, Chief Custodian Constantin Valdor and the Masters of the Adeptus Astronomica, the Adeptus Astra Telepathica and the Administratum of the Imperium were also appointed to the Council.

Having established the new governing body of the Imperium to carry out the day-to-day work of ruling tens of thousands of worlds and trillions of human beings, the Emperor took refuge in his vast laboratories and workshops beneath the Imperial Palace. He began work in earnest on his new project. While the Emperor was locked away in his subterranean factories, trouble was brewing. The formation of the Council proved to be a contentious decision with the distant Primarchs, who were appalled when news of the formation of the Council of Terra finally reached them on the frontiers of the Great Crusade. Some of the Primarchs took great exception to being ruled by those they deemed less worthy of such an honour than themselves. The less stable Primarchs felt that this was a betrayal of all they had fought and won in the Emperor's name and that their victories now counted for nothing. Some of the Primarchs took great exception to being ruled by those deemed less worthy of such honours than themselves. They, and many of their Astartes, felt that it was they who had suffered and sacrificed the most to build the Imperium and thus it was they who should have the greatest say in how it was ruled, not a council composed of effete Terran nobles and faceless bureaucrats. This was one of many growing resentments that allowed the Ruinous Powers to infect and corrupt several of the Primarchs.

The creation of the Council of Terra, seen in this light, lent new weight to Horus' argument to several of his brothers that the Imperium had been betrayed by their father. He argued that the Emperor had proved more than willing to turn his back on his sons and generals and give power instead to petty mortal administrators and the sycophantic Tech-adepts of Mars who lacked the Primarchs' brilliance and superhuman abilities. As Horus prepared his rebellion against the Emperor, he convinced himself that petty functionaries and administrators had supplanted the Primarchs and the Astartes within the Imperium they had won. Once the Imperium had been wholly geared for the war and conquest that was its life's blood since its inception, but now it had become burdened with parasitic exectors, scribes and scriveners who demanded to know the cost of everything. Bureaucracy was taking over -- red tape, administrators and clerks were replacing the heroes of the age. Horus argued to hs more receptive brothers that unless the Imperium learned to change their ways and their direction, their greatness as an empire would soon be a footnote in history books. Horus feared that everything he and his brother Primarchs had achieved would be a distant memory of former glory, lost in the mists of time like the civilisations of ancient Terra. It was this hubris and arrogance that led to the Warmaster's inevitable fall to Chaos and the resultant civil war that would consume the entire galaxy, ushering in a new Age of Darkness that would last for millennia.

Yet Horus and his more resentful brothers had completely misunderstood the Emperor's intent in creating the Council of Terra. The Council was to become the body of civilian government that would administrate the myriad bureaucratic tasks needed for the survival of the newly formed human interstellar empire. The Emperor was determined that in his Imperium power would reside with those men and women who were governed by its apparatus and not with an artificial military elite composed of genetically-engineered beings who were so powerful that they already possessed only a very tenuous grip upon their own humanity. The Primarchs and their Space Marines had been created to give life to the Emperor's dream of a united human Imperium stretching across the galaxy and to defend it from humanity's myriad foes. They were not to rule it as a hereditary caste of immortal warriors imposing their whims upon those they deemed mere "mortals" by brute force.

The Corruption of the Astartes Legions
During the Great Crusade, it became apparent that the Primarchs were far from the perfect specimens of humanity they were intended to be. Although each Primarch was physically and mentally god-like compared to a baseline human being, they harboured the flaws of vanity, egotism, hunger for power, jealousy, arrogance, insecurity and all the other sins of the human character.

As the 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 administration of the Imperium. Only a handful of the Primarchs, among 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, 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 Webway Project. These seeds of bitterness, resentment and frustration grew, and would soon bear deadly fruit.

It was on the moon of the world of Davin that Horus' fate was sealed. This was the second time his Legion had been posted to this world; after the previous visit sixty years earlier the Luna Wolves had adopted the native Davinite institution of warrior lodges. Though these lodges had begun as simple fraternities of warriors, their secretive nature handed Lorgar, the Primarch of the Word Bearers Legion and his First Chaplain Erebus, the tool they needed to manipulate Horus towards the service of the Chaos Gods.

Lorgar and his Word Bearers originally came from Colchis, a world defined by religious fanaticism. They had long worshiped the Emperor as a god. The Word Bearers had sought to spread their Cult of the Emperor to every world they added to the Imperium. But the Emperor deeply disliked and mistrusted organized religion (ironic, since he had often been the focus for much of it in his various guises across history), blaming it for much of the darkness that had plagued humanity's history. The Emperor openly and publicly refuted his alleged divinity and banned religious worship in his empire, and demanded that his subjects accept the "Imperial Truth"-- that science, reason and logic alone presented the tools required to create a better human future. Lorgar did not suffer the Emperor's reprimand or views on religion well. Angered and wounded that the Emperor would not accept his devotion and worship, Lorgar turned instead to the Ruinous Powers of the Warp -- who were all too willing to accept the devotion of one of humanity's Primarchs. Before long, the Word Bearers Legion had been almost entirely corrupted by the Chaos Gods, and Lorgar and Erebus were tasked by the Ruinous Powers with corrupting all of their fellow Space Marines -- starting with the greatest of them all, the Warmaster Horus.

During a battle against Chaos-spawned undead on Davin's moon, whose Planetary Governor, Eugen Temba, had been corrupted by the forces of the Chaos God Nurgle, Horus was poisoned by a xenos blade dedicated to Nurgle known as a Kinebrach Anathame that had been stolen from the human civilisation of the Interex by Erebus after Horus and the Luna Wolves of the 63rd Expeditionary Fleet had made a disastrous first contact with them. Erebus then gifted the weapon to the Chaos-corrupted form of the Imperial Army commander Eugen Temba who the Warmaster had left behind to govern Davin sixty years before. Temba had turned to the worship of Nurgle in the interim, being transformed into a bloated mutant and killing off most of his Imperial Army garrison in the process, transforming them into undead Plague Zombies that the Luna Wolves were forced to mow down in waves. Horus personally faced off with the mutant that had been Temba aboard the grounded ruins of his Imperial Cruiser. In the course of that battle, the potent living metal of the Chaos blade wielded by the plague-infused monstrosity left Horus with a bleeding, toxic wound in his shoulder that his Legion's Apothecaries could not heal despite all the advanced technology available to them. Seeing his chance to further the designs of Chaos, Erebus next persuaded the Luna Wolves' warrior lodge to allow a group of Davinite shamans -- Chaos Cultists all -- located on the surface of Davin at the Temple of the Serpent Lodge to heal him. The Luna Wolves, besides themselves with grief and the fear that their beloved Primarch would die, agreed to the suggestion, despite its direct violation of the creeds of the Imperial Truth.

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 worshiped 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 of 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. The Emperor ordered the Primarch Leman Russ, Magnus' greatest rival, to mobilize his Space Wolves Legion and the witchhunters 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 sorcery 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 realized he was being manipulated by the entities that called the Immaterium home.

The Istvaan Massacres
Note: Published materials are inconsistent on their spelling of "Isstvan"; the more recently published material uses "Isstvan", while other (generally older) materials use "Istvaan". No explanation for this difference has been provided. For the sake of expediency, this article will use the spelling Istvaan, with no claims made to the accuracy of the spelling.

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 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 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 Prospero and its famed libraries 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.

Istvaan III Atrocity
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.

Flight of the Eisenstein
Seventy Loyalists of the Death Guard Legion and the Imperial Saint Euphrati Keeler led by Battle-Captain Garro had commandeered the Imperial frigate Eisenstein and, evading the Traitor forces of Horus, were able to escape from the Istvaan System into the Immaterium, after being told what was happening on the planet. The Eisenstein was badly damaged by the Death Guard battleship Terminus Est during its escape from Istvaan III and it was assaulted by undead minions, including the first known Plague Marines of Nurgle while it was within the Immaterium. This assault by Warp entities forced the ship to make a crash emergence from the Warp. The repeated traumas left all of the frigate's astropaths dead, and its lone Navigator was mortally wounded. However, Garro managed to attract the attention of passing Loyalist starships by setting the vessel's Warp-Drives to self-destruct and ejecting them from the starship. Rogal Dorn's Imperial Fists Legion's massive mobile fortress-monastery Phalanx and the Legion's fleet had been becalmed in the Warp for some time due to the waxing power of the Ruinous Powers as the Heresy began, and his Navigators sensed the detonation of the Eisenstein's Warp-Drives. Charting an immediate course for the location of the detonation, Dorn met with Garro, who explained to him all that had happened with the Traitor Legions. Dorn was reluctant to believe Garro's tale, but overwhelming proof from a Remembrancer (journalist) named Mersadie Oliton who had escaped from Horus' flagship, the Vengeful Spirit, and Garro's dogged insistence finally convinced the Primarch. The Phalanx fortress set a course for Terra.

The fate of the Eisenstein's survivors is unknown. Sequestered on Luna in a tower belonging to the Sisters of Silence after their arrival in-system, Garro, the rest of his Loyalist Death Guard, Captain Iacton Qruze of the Luna Wolves, and the warriors of the Silent Sisterhood faced one of the Death Guard Marines named Solun Decius who had succumbed to the temptations of the Chaos God Nurgle after being infected with one of the Lord of Plague's more virulent illnesses now known as Nurgle's Rot by the undead minions Nurgle had unleashed on the Eisenstein. Seeking to end his pain, Decius allowed himself to become possessed by a Lesser Daemon of Nurgle, a variant Plaguebearer called the Lord of the Flies. Afterwards, Garro and Captain Qruze were met by Malcador the Sigillite, leader of the Council of Terra and the Emperor's Regent, who informed them that the Emperor had need of people who were strong of will and as "inquisitive" as they. Malcador ordered Garro to begin to comb the galaxy for 7 other Astartes from both the Traitor and Loyalist Legions who with him would become the founding members of the Imperial Inquisition and the militant arm of its daemon-hunting Ordo Malleus -- the Grey Knights.

The Drop Site Massacre on Istvaan V
In response to Horus' betrayal 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 7 Loyalist Space Marine Legions to Horus' base on the world of Istvaan V to challenge the Warmaster. 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, the Salamanders, and the Raven Guard. The Legions comprising the second 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.

The Iron Hands, Salamanders and Raven Guard were deployed in the first wave of the assault. After they secured the drop site, they were to have been followed by the arrival of the other four Legions. The first wave secured the drop site at heavy cost. Horus ordered his front line troops to fall back, tempting Ferrus Manus to overstretch his already thin lines. Against the advice of Corax and Vulkan, Manus led his Veterans against the fleeing Traitor Marines unsupported. Manus then brought his brother Fulgrim to combat. As the two Primarchs drew their weapons, the Raven Guard and Salamanders fell back to regroup and allow the second wave's Legions to advance and earn glory. However, as they returned they were mowed down by the four Traitor Legions that had landed to supposedly support them, thus revealing their new allegiance to Chaos. Horus then pressed his own attack and sandwiched the Loyalists between the two Traitor forces, killing most of them. Several determined groups of Loyalist Space Marines managed to break out of the trap and escape back to the few remaining Loyalist drop ships.

Ferrus Manus was beheaded in the battle by Fulgrim. Fulgrim was beset by grief as he stared down at his once beloved brother's headless body; in this moment of weakness as the Emperor's Children Primarch repudiated his allegiance to Chaos, the Greater Daemon of Slaanesh within his potent blade tricked him into lowering his mental shields, and possessed him, crushing Fulgrim's soul into the recesses of his mind, a mute witness to the daemon's actions. The daemon would present Ferrus Manus' head to a delighted Horus, though even the Warmaster was disgusted with the creature possessing his brother's body.

Barely a handful of Loyalist Space Marines escaped with their lives from the Drop Site Massacre of Istvaan V to bring word of the further betrayal of four more Astartes Legions to the Emperor. A critically wounded Corax was brought back to Terra in a stasis tube. Vulkan was missing, presumed dead, though he would later reemerge after a harrowing journey back to Terra to lead his Legion once more. Vulkan was one of the Primarchs who protested Roboute Guilliman's publication of the Codex Astartes and the splitting of the remaining Space Marine Legions into 1,000-man Chapters following the death of Horus and the end of the Horus Heresy.

The Route to Terra
After the Drop Site Massacre, it became clear that 8 of the 18 Space Marine Legions had turned to Chaos and against the Emperor of Mankind. Horus openly and publicly declared that he would no longer follow the Emperor, believing him to be undeserving of the battles fought in his name, and the Warmaster took over the formal leadership of the Traitor Legions as their new overlord. His Traitor Legion forces were supported by elements of the Imperial Army, a portion of the Adeptus Mechanicus and the Titan Legions corrupted by Chaos that were known as the Dark Mechanicus, strong Imperial naval forces then still part of the Legiones Astartes' Great Crusade Expeditionary Fleets, and the daemon-spawn of Chaos.

Over the next seven Terran years, the combined Traitor fleets spread across the galaxy, unleashing a brutal civil war upon many of the planets that they had once helped to bring into the Imperium. Secretly guided by the Ruinous Powers, the Traitor Legions and their related support forces no longer worried about the rules of war, civilian casualties or proportionality; it was a common tactic of the Traitor fleets to simply unleash a total orbital thermonuclear bombardment upon a single inhabited world in a heavily defended Imperial star system to induce all of the other worlds and orbital installations to surrender. Casual brutality and even genocide became the norm for the Traitors as they fell ever more deeply under the control of Chaos. Slowly but methodically, Horus and the Traitor Legions forged a path across the galaxy lit by the burning nuclear pyres of countless worlds. Those planets spared the destruction faced an even worse fate: occupation and control by the Chaos-corrupted Traitor forces, who took great pleasure in unleashing the perversions of the Dark Powers upon the captive populaces. Many prayed to the Emperor for release from their torments, and the strength of the Lectitio Divinitatus cult that believed in the Emperor's divinity only began to grow as the nightmare of civil war scourged the Imperium of Man.

It is not entirely clear how Horus managed to turn such a significant percentage of the armies under his command against the Emperor, but he was known to be a very skilled and persuasive leader who commanded immense personal loyalty amongst his subordinates. But even before the opening stages of his planned insurrection occurred, he knew he would have to secure the support of the Adeptus Mechanicus and their superior technology and weapons if he was to defeat the Emperor and conquer the galaxy. Horus won over the loyalty of many of the Mechanicus' Tech-Adepts after promising them the lost secrets of ancient Standard Template Construct (STC) technology that had been recovered from the worlds of the recently subjugated Auretian Technocracy by the Sons of Horus Legion.

The Heresy also struck at the technological and industrial heart of the Imperium on the Forge World of Mars, the sacred homeworld of the Adeptus Mechanicus. The climate on Mars was full of discontent during this tumultuous time. There were tense relations between the various Techno-Magi with sporadic outbreaks of espionage and violence being committed against the various forges that represented the primary sociopolitical units of Mars. There were even unconfirmed suspicions that the Titan Legions had already secretly chosen sides in case of a potential conflict. Regulus, the Adeptus Mechanicus' representative to the Warmaster Horus who had already thrown in his lot with the Warmaster, was sent to Mars to secure the tentative support of the Fabricator-General of Mars, Kelbor-Hal. Regulus convinced the Fabricator-General of the Warmaster's resolve to support increased autonomy for the Mechanicus under his rule rather than maintaining the more autocratic mandates of the Emperor that prevented the Mechanicus from doing exactly as it pleased. As a show of his appreciation for the Fabricator-General's support, Horus provided information to Kelbor-Hal that allowed the Mechanicus to open a repository of forbidden knowledge known as the Vaults of Moravec, which had been sealed for a thousand years. The Emperor himself had decreed that the vaults never be opened, for they contained innumerable artefacts of technology that had been fashioned or corrupted by the malign power of Chaos. But the deal was struck,and the Fabricator-General accepted Horus' proposal and joined forces with the Warmaster, assisting the Traitors with all of the technology of Mankind at his disposal. When this repository was reopened, there was all manner of forbidden arcane knowledge and weaponry that had obviously been tainted by the corrupting influence of Chaos stored within. Soon the corruption spread throughout the forges and temples across the Red Planet as scrap code -- Chaos-contaminated digital source code that was infected with an arcane computer virus -- infested the logi-stacks and Cogitator (computer) archives of the Adeptus Mechanicus, causing literal Chaos to emerge in any Cogitator system that was networked to one of its infected counterparts. The Fabricator General and his Dark Mechanicum allies used this disruption to marshal the strength of their forces, intent on bringing the rule of Mars firmly under their control. Many Mechanicus forces that remained loyal to the Emperor as the Omnissiah were forced off-line or destroyed outright by the traitorous Fabriactor-General's ploy. In later years, the night the Chaos code was released would become known in Mechanicus legends as the Death of Innocence.

Open warfare erupted on the Red Planet in the wake of the Death of Innocence as Martian forces, both civilian and military, fought one another in a deadly civil war that mirrored the larger conflict consuming the Imperium. Across the galaxy, open rebellion erupted as Space Marine Legion fought Space Marine Legion and Titans battled Titans. The members of what became known as the Dark Mechanicus supported Horus throughout the Horus Heresy and used dark and forbidden knowledge revealed by the Ruinous Powers to help destroy the Loyalist forces.

As the years of conflict across the galaxy passed, the ultimate aim of the Traitor Legions' destructive interstellar advance soon became clear: their goal was the conquest of Terra, the heart of the Imperium. With this conquest, Horus could fulfill his promise to the Chaos Gods and throw down the Emperor of Mankind, their greatest enemy. The Sons of Horus, the Death Guard, the Emperor's Children, the World Eaters, and elements of the Word Bearers prepared to rendezvous at Mars. The rest of the Word Bearers Legion was tasked with destroying Roboute Guilliman and his Ultramarines on the world of Calth in the Eastern Fringe of the galaxy.

The Imperial Fists Legion's Primarch Rogal Dorn and the Regent of the Imperium, Malcador the Sigillite, receiving the few survivors from the Drop Site Massacre of Istvaan V as they retreated back to Terra, became aware of the full implications of their position and the strength of Horus and the Traitor forces. Dorn immediately recalled all Loyalist Imperial forces back to Terra in preparation for Horus' invasion. Of the Space Marine Legions still loyal to the Emperor, the Space Wolves had just completed the Burning of Prospero, the Thousand Sons Traitor Legion's homeworld near the Chondax System, where the White Scars were stationed. Without warning, the Alpha Legion's fleet broke from the Warp and engaged the Space Wolves, hammering their smaller fleet and forcing Leman Russ to resort to hit-and-run attacks. The Alpha Legion's Primarch Alpharius also attacked the nearby forces of the White Scars piecemeal in an attempt to draw that larger Astartes Legion into conflict.

The White Scars' Primarch Jaghatai Khan wished desperately to aid Russ, yet as the Traitor Legions' starships attacked, he received the astropathic order from Rogal Dorn. Khan was to bring his Legion back to Terra, immediately. Dorn also commanded him to relay the order to Russ and add that should he succeed in evading his attackers, only then should he attempt to head for Terra. Relaying the message and adding his apology for his inability to aid the Space Wolves, the White Scars made for Terra. Russ resolved to meet the Alpha Legion with renewed determination. With help from an unlikely quarter, the Space Wolves would eventually turn the tables on their attackers and make the Warp jump to Terra, well after the siege on the Imperial Palace had begun.

Similarly at the world of Calth, the Ultramarines expeditionary force, battered by the relentless attacks of the Word Bearers, had dug in on the planet's surface, while their Primarch Roboute Guilliman and the remnants of his fleet began to organise hit-and-run attacks. Surveying the scene on the planet, Guilliman rapidly assessed his ground troops' positions, and began broadcasting orders to his men, coordinating each pocket of defence. One such pocket, under Brother-Captain Ventanus, organised a breakout, and retook Calth's planetary defence cannons and laser silos, reaping a great tally of Traitor starships. Ventanus' victory evened the odds in space, buying time for the vast remainder of the Ultramarines Legion to arrive at Calth and drive the Traitors away. Reunited as a single force, the Ultramarines received Malcador's orders and set out for Terra, moving through the Warp as rapidly as they could even as they realised that they would probably arrive too late to make a difference.

As this happened, the Night Lords arrived in the Eastern Fringe of the galaxy to engage the Dark Angels, and the Iron Warriors' armada broke from the Warp to engage the Imperial Fists' fleet marooned near the Traitor Legions' headquarters world of Istvaan V. Surviving the initial thrust of the Iron Warriors' attacks, the Imperial Fists' armada held fast and scattered Perturabo's fleet, then made their own Warp jump to Terra in a desperate race against time.

Meanwhile, in the Signis Cluster, the Blood Angels, granted new and terrible power by a mysterious mass rage that would resurface again during the Battle of Terra had triumphed, smashed the hordes of the Chaos daemons they had faced asunder and were able to make the Warp jump to Terra.

As the Warmaster was heading towards Terra he received an unexpected communication from the recently betrayed Thousand Sons Primarch Magnus the Red. Magnus pledged his allegiance and the allegiance of the Thousand Sons Legion to Horus and Chaos in retaliation against the Emperor for his betrayal of the Thousand Sons and the destruction of their homeworld of Prospero. The Thousand Sons informed the Warmaster that they were en route to Terra where they would link up with Horus' forces.

Of the nine remaining Loyalist Space Marine Legions, only the White Scars and the Blood Angels were able to join Rogal Dorn and his Imperial Fists in the defence of Terra before the arrival of the Traitor Legions in the Solar System. Three entire Titan Legions of the Adeptus Mechanicus and close to two million soldiers of the Imperial Army stood alongside them to face the Traitors in a battle that would determine the fate of Mankind for 10,000 years.

The 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.

The Siege


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'bandah 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.

The 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 Adeptus 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 Chaosaboard 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 the Warmaster's 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 Battle Barge, 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 Adeptus Custodes warrior entered the bridge. Horus showed the Imperial bodyguard the Emperor's broken form and laughed at the Custode. The valiant Imperial warrior roared and charged the Warmaster. He was flayed alive by a glancing psychic blast of Chaotic power from Horus. In other accounts of this final battle in some Imperial sources, an Imperial Fists Terminator attacked Horus; in still other versions, the doomed man is an Imperial Armytrooper named Ollanious Pious. 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 defenses and ripped his body apart. In some versions of the tale, this blast was only able to pierce the Warmaster's 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 him 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 Traitor Space Marines and Titans fell attempting to flee. The ground before the Sanctum Imperialis ran red with the blood of Traitors and Heretics.

Meanwhile, the Primarch Rogal Dorn finally found his way to the corrupted starship's bridge, only to discover his fallen brother, Sanguinius, and the shattered body of the Emperor, who was now on the verge of death, his remaining psychic energy spent in the battle with Horus. It was then that the Emperor whispered instructions to Dorn, urging the Imperial Fists' Primarch to take him to the device known as the Golden Throne in the inner sanctum of the Imperial Palace. The surviving Loyalists teleported back to the Imperial dungeons beneath the Imperial Palace. Here Malcador the Sigillite, the Regent of Terra who was himself an extremely powerful psyker, had briefly taken the Emperor's place on the Golden Throne, thus keeping the Warp Gate to the Webway that the Emperor had been building for the benefit of humanity safely closed from an assault by the daemons of the Realm of Chaos. Malcador's body collapsed to dust from the sheer strain of holding back the dangers of the Immaterium from coming through the Webway gate and assaulting Terra as he was removed from the Throne and the Emperor put in his place. Yet, before his death, the Sigillite imparted his remaining psychic energy to the Emperor. This sacrifice restored the Master of Mankind's consciousness so that he could deliver his final instructions to his servants before being interred in the device as the Tech-priests of the Adeptus Mechanicus worked furiously to modify the Golden Throne to the Emperor's specifications.

The Emperor then spoke his final words to his followers. He urged them to continue the fight to free humanity from the Forces of Chaos and the machinations of the Ruinous Powers as well as the ignorance that continued to assail it. And then the Master of Mankind spoke no more, his body entombed within the life-support mechanisms of the Golden Throne, his spirit caught between the Warp and a crippled, decaying body for millennia. Unable to complete the Webway project that the Golden Throne had originally been intended to create and maintain, the Emperor now chose to use its psychic augmentation mechanisms both to maintain his shattered body's life support functions and to project his mind into the Immaterium where he would create and maintain the Astronomicon navigation beacon to hold the Imperium together. He would also battle the Chaos Gods on their own plane, always seeking to protect Mankind from their corruption and depredations. The Imperium of Man had survived the Horus Heresy intact, but would evolve into the bastion of repression, ignorance and brutality the Emperor had long fought against without his direct, guiding hand upon it. In this, at least, the Chaos Gods had won a partial victory, for this state of affairs would ensure that their temptations would continue to corrupt the weaker members of the human race and prevent the final establishment of Order across the galaxy. Yet, for all its terrible and growing flaws over the next ten millennia, the Imperium would also offer humanity its best and perhaps only hope for survival in an uncaring and increasingly hostile universe.

The Great Scouring and the Reformation
As the flames of the civil war subsided across an Imperium in which 2.3 trillion human beings had died and countless worlds had been utterly shattered, the Ultramarines' Primarch Roboute Guilliman rallied the surviving Loyalists, stretching his own Legion thin across the galaxy to buy time for the other Space Marine Legions to rebuild and rearm to take the fight to the remaining Chaos Space Marine Traitor Legions in what became known as the Great Scouring. Although some Traitor Legions had fled to the relative safety of the Eye of Terror, the Iron Warriors continued to hold fast to the worlds they had conquered, and were dislodged only after decades of gruelling warfare. Similarly, the Chaotic Night Lords Legion continued their spree of genocide and terror along the Eastern Fringe of the galaxy. This campaign endured until the Night Lords' Primarch Night Haunter was assassinated on the world of Tsagualsa. Shortly after, Roboute Guilliman slew the Alpha Legion's Primarch Alpharius (but not his secret twin brother Omegon) on the planet of Eskrador.

As the last of the Traitor Legions fled to the safety of the Eye of Terror at the end of the Great Scouring, Roboute Guilliman turned his attention to rebuilding the Imperium of Man itself in what became known to later Imperial historians as the Reformation. Never again would it succumb to a Heresy so vast or destructive. Guilliman divided the Imperial Army into the separate branches of the Imperial Navy and the Imperial Guard, diluting their ability to mutiny en masse by creating a bureaucratic rivalry between the services. The Imperial Inquisition had been founded by Malcador at the instigation of the Emperor at the start of the Heresy to prevent the secret spread of the corruption of Chaos through the ranks of humanity from happening again, and the Grey Knights Space Marines Chapter was later founded from the core group of 8 Astartes gathered by Nathaniel Garro to combat daemonic incursions as part of what later became the Inquisition's Ordo Malleus. The Council of Terra that had been created by the Emperor after the Ullanor Crusade to provide the Imperium with a civilian administration was succeeded by the High Lords of Terra: a Senatorum Imperialis composed of the 12 most powerful individuals in the Imperium who were the leaders of its various governmental institutions known collectively as the Adeptus Terra. The High Lords were to act as the regents of the Imperium and rule in the Emperor's name as they sought to carry out his will.

Of all the changes brought upon the Imperium by Roboute Guilliman after the Horus Heresy, the most pivotal was the Second Founding of the Space Marines. This reform, enacted seven standard years after the defeat of Horus, saw the remaining Loyalist Space Marine Legions broken up into smaller, more flexible 1000-man units known as Chapters, thus transforming the Legiones Astartes into the Adeptus Astartes. Although Rogal Dorn, Vulkan, and Leman Russ opposed breaking up their beloved Legions, Guilliman's initiative had the backing of Corax, Jaghatai Khan and the High Lords of Terra, and the measure passed irregardless of the other Primarchs' opposition, marking the end of the Primarchs' dominance over Imperial life and the imposition of mortal rule that the Emperor had desired. The Adeptus Astartes were granted almost total autonomy from every other Imperial Adepta, including the Inquisition, but not from the High Lords of Terra who spoke for the Emperor. Never again would one man wield the power of an entire Space Marine Legion or have such total control over the Imperium. At the same time, Roboute Guilliman completed his great work, the Codex Astartes, the famed tome that laid out the proper organisation, tactics and order of battle for a Space Marine Chapter. The Codex soon became a sacred tome for the Ultramarines and their myriad Successor Chapters, the backbone of the Adeptus Astartes in the centuries after the Horus Heresy. Other Chapters, like the Blood Angels, the Dark Angels and the Space Wolves, adopted the Codex's guidelines only in part or not at all, a sign of their continuing disagreement, however ineffective, with the reforms of Guilliman.

Continuity
Beginning in 2005, the story of the Horus Heresy has started to be expanded with a collection of art books and novels created by various authors from the Black Library, set in the time period of the early 31st Millennium as opposed to the circa 40,000 AD era of normal Warhammer 40,000 literature. An initial scene-setting trilogy featuring the Luna Wolves and portraying the fall of their Primarch, the Warmaster Horus and the beginning of the Heresy through the eyes of the 10th Company Captain of the Sons of Horus Legion Garviel Loken was released and since the completion of said trilogy, another series of novels followed Battle-Captain Nathaniel Garro of the Death Guard Legion as he created what would become the core of the Grey Knights and the Inquisition. The novel range has expanded to include the actions of other Astartes Legions during the terrible conflict and the role of the various Imperial Adepta. In some cases the new material introduced in the card game, art books and novels has led to continuity conflicts with the older material, although the information provided in the novel series is generally assumed to be canonical and superseding the older material when a conflict arises, and is the basis for the outline of the Horus Heresy given here.