Horus Heresy

The Horus Heresy was a galaxy-spanning civil war that marked the end of the Great Crusade. First described in the Realm of Chaos: Slaves to Darkness supplement it was used as the background for the original "Epic-scale" Space Marine and Adeptus Titanicus games, providing a justification for inter-Imperial warfare and thus spared the effort to include different armies in the basic game boxes. "Horus Heresy" is also the title of a novel series published by the Black Library, a collectible card game produced by Sabertooth Games and an out-of-print Games Workshop board game, with both games being based on the events which occurred in the Warhammer 40,000 universe.

Overview
Recently, as of 2005 the background material has started to be expanded with a collection of art books and novels created by various authors from the Black Library, set in this time period 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 captain of the legion Garviel Loken was released and since the completion of said trilogy, after Garviel Loken another series of novels followed Captain Garro of the Death Guard. The novel range has expanded to include the actions of other legions during the war and the role of various Imperial organisations; as of the present day further novels are still being added to the series, and the novels have only covered up to the massacres of Istvaan V in the traditional timeline of the Heresy.

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.

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, the Emperor deemed it time to begin his Great Crusade, a massive galactic campaign by which he and his armies would free all human-settled worlds from oppression and unite 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, however, when they were snatched away from the special laboratory where they had been developed. The cause of this remains debatable; some argue that the Emperor sent the Primarchs away so that they could learn in their own way, others argue that the Chaos Gods broke into the laboratory and, unable to destroy the Primarchs, instead chose to scatter them across the galaxy, where they eventually came to rest on diverse, human-inhabited worlds. The later theory would seem to be confirmed by Games Workshop fiction, and is generally regarded as canon or semi-canon by most fans.

During the Great Crusade, the Emperor encountered each of the Primarchs in turn. A Space Marine Legion had been created using each of the Primarchs' genetic material, and so the Emperor, unable to be everywhere at once, deemed it fitting that each Primarch should lead their genetic offspring. However, this would prove a critical mistake, as the Space Marine geneseed creation process made the Emperor, the Primarchs and the Space Marines analogous to Grandfather, Father and Son. In time, many of the Marines in the Legions would come to venerate their Primarch more than the Emperor.

After 200 years of hard conflict in the thirty-first millennium, over two million worlds had been reclaimed by the Emperor in the name of humanity. Beside him stood the Primarch Horus, who had fought beside the Emperor for the early part of the Great Crusade as his only rediscovered son. The long wars had forged a strong bond between them, and they were truly like father and son. But now the Emperor had to consolidate his new Imperium, and undertake the next phase of his Grand Plan. This 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". Horus was now the Commander in Chief of all 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 didn't understand why the Emperor was leaving them to fight alone and, worse still, why Horus should be raised over all of the other Primarchs to take command over them. Rogal Dorn, Sanguinius and Fulgrim were pleased for their new Warmaster, while others - such as Angron, Roboute Guilliman, Lion El'Jonson and Perturabo - all reacted with varying degrees of disapproval.

The Corruption of the Legions
During the Great Crusade, it became apparent that the Primarchs were far from the perfect beings they were intended to be. Although each Primarch was physically and mentally god-like compared to a baseline human being, their personalities were each as flawed as those of any other mortal. During their upbringing on their respective homeworlds, the Primarchs had to learn humanity from mere humans; for almost all of the Primarchs, this resulted in their harbouring all-too-human flaws (for specific examples, see each Legion's history).

Horus took over command of the Great Crusade, and took up his new duties with earnest dedication. However, there was much dissension in the ranks of the Primarchs and other parties. Only a handful of the Primarchs, among them a scheming Lorgar, remained steadfast beside the Warmaster during this period of dissension. Horus also disagreed with many of the new 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 that he was failing his father, and was deeply wounded that the Emperor had revealed to none of the Primarchs, not even his most favored son, why he had secluded himself upon Terra. These seeds of bitterness, resentment and frustration grew, and would soon bear deadly fruit...

Meanwhile, the Emperor of Man was on Terra organizing the infrastructure for his Imperium to function. He had created the Council of Terra, a body of bureaucrats and nobles that would implement and administer the new galaxy-wide tax called the Imperial Tithe and other matters of day-to-day law in the Imperium of Man.

The news of the creation of the Council of Terra and these latest bureaucratic edicts angered some of the Primarchs still further. They did not understand why they, the Emperor's greatest champions, who had spilled their blood on a thousand worlds to re-unify all the races of Man, did not have seats on this new Imperial ruling body. The brotherhood of the Primarchs was being shattered bit by bit by this growing resentment and jealousy. Old arguments and differences came to the fore. Horus became ever more distant from the Emperor, seeking only glory for himself and his Legion.

It was on the moon of the world of Davin that Horus's 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. Lorgar and his Word Bearers came from a world of religious fanaticism and 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 deply disliked and mistrusted organized religion, 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 'Imperial Truth'-- that science and logic alone presented the tools to create a better human future than any civilization had managed in the past.

Lorgar did not suffer the Emperor's reprimand 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 his First Chaplain were tasked by the Ruinous Powers with corrupting all of their fellow Space Marines--starting with the greatest of them all, the Warmaster Horus.

On Davin's moon, which had been corrupted by the forces of the Chaos God Nurgle, Horus was poisoned by an anathame stolen from the Interex by Erebus and gifted to the Chaos-corrupted form of the Imperial Army commander (Eugen Temba) the Warmaster had left behind to govern Davin sixty years before. The potent living metal of the blade left Horus with a bleeding wound in his shoulder that his Legion's apothecaries could not heal. Seeing his chance to further the designs of Chaos, Erebus persuaded the Sons of Horus' warrior lodge to allow a group of Davinite shamans, known as the "Serpent Lodge" located on the surface of Davin at their temple called Delphos - Chaos cultists all - to heal him.

During the rituals, Horus' spirit was transferred into the Warp. 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 his Primarchs (but not Horus) were worshiped as living gods by the masses(ironically,this is exactly what happened after Horus' death, though it was an outcome largely caused by the Warmaster's own actions}. The Chaos 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, Primarch of the Thousand Sons Legion, had also traveled 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". Horus accepted the Ruinous Powers' offer. They healed his grievous wound and charged him with the powers of the Warp. Renouncing his oath to the Emperor, Horus led his Legion, the Sons of Horus, into worship of the myriad Chaos Gods. He then sought to turn many of his fellow Primarchs to 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, Primarch of the Thousand Sons Legion, foresaw Horus's actions through his Legion's own use of forbidden psychic sorcery. Magnus then attempted to forewarn the Emperor of the impending betrayal. However, knowing that he would have to find a means of quickly warning the Emperor, and as an act of both desperation and vindication, Magnus used sorcery to send his message to the Emperor. The message penetrated the 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, the creation of a warp-gate to invade the Eldar's Webway. It was this project that the Emperor had returned to Terra to complete, leaving his Primarchs to complete the Grand Crusade alone. 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 Legion. The Emperor ordered the Primarch Leman Russ to mobilize his Space Wolves Legion and take Magnus into custody; however Horus convinced Russ that Magnus was a traitor and needed to be destroyed.

The Istvaan Massacres
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' success arose from the thorough groundwork he had laid before the opening shots of the Heresy were fired at Istvaan. He had already swayed Angron and Mortarion. Lorgar, who had been responsible for the budding rebellion and Horus turn to 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 far from Terra and Istvaan. 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 Dropsite Massacre, Fulgrim also attempted to sway his friend Ferrus Manus to Horus' cause, failed, and barely escaped with his life. Fulgrim promised he would deliver Manus's severed head to Horus in recompense. The Blood Angels were sent to the daemon-infested Signis Cluster and the Ultramarines to Calth, where a large Word Bearer force, under Kor Phaeron, had massed.

Of the other eventual Traitor Marines, Konrad Curze was due to face disciplinary action from the Emperor; the Alpha legion primarch Alpharius had always been closer to Horus; and the Iron Warriors Primarch Perturabo's bitterness towards Rogal Dorn made him an easy target for corruption. The Thousand Sons had never planned to join Horus, but the path Tzeentch had mapped for the Thousand Sons led them to Chaos regardless.

The remaining Space Marine Legions - the Raven Guard, Salamanders, Iron Hands and Space Wolves - remained staunchly loyal to the Emperor, though all but the Wolves would pay dearly for it in the battles to come. Beyond the Legions, Horus had already swayed Adept Regulus with promises of the STCs recovered during the war with the Auretian Technocracy, delivering Adeptus Mechanicus support to the Warmaster's Traitor Marine forces.

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

Istvaan III
The first sign that Horus and his Sons of Horus Legion had turned to Chaos was made evident when Horus virus-bombed the rebel world of Istvaan III. Unknown to the Emperor, the Word Bearers had been devoted to Chaos for some time before this event. The Planetary Governor of Istvaan III, Vardus 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. Although the four Legions under his direct command had turned Traitor, there were still some Loyalist elements within the Sons of Horus, World Eaters, Emperor's Children and Death Guard; many of these were Terran Space Marines who had been directly recruited by the Emperor before being reunited with their Primarchs during the Great Crusade. Horus, under the guise of his orders, amassed his troops in the Istvaan System.

Horus had a plan by which he would destroy all Loyalist elements of the Legions under his command. After a lengthy bombardment, Horus dispatched all of the known Loyalist Marines down to the planet, with the pretense of bringing it back into the Imperium. At the moment of victory and the capture of Choral City (once a very elaborate and beautiful city), the planetary capital of Istvaan III, these Marines were betrayed when the virus bombs began to fall. Saul Tarvitz of the Emperor's Children, however, was aboard one of the orbiting ships and discovered the plot to wipe out the Loyalist Marines. He was able, with help from Battle-Captain Nathaniel Garro of the Death Guard, to reach the surface of Istvaan III and warn the Marines he could find of their impending doom. Those that heard took shelter before the virus bombs struck. The population of Istvaan III received no such protection: Twelve billion people died almost at once. The psychic shock of so many deaths at one time shrieked through the Warp, briefly obscuring the Astronomican. Angron, realising that the virus bombs had not been fully effective, flew into a rage and hurled himself at the planet with 50 companies of Traitor Marines. Discarding tactics and strategy, the Legion 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 Nathaniel Garro of the Death Guard 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 Tarvitz, Loken and Torgaddon, fought bravely against their own traitorous brethren. Despite some early successes, their cause was doomed. During the battle Ezekyle Abaddon and Horus Aximand were sent to confront their former Mournival brothers, Loken and Torgaddon. Horus Aximand beheaded Tarik Torgaddon, but Abaddon failed to kill Loken when the building they were in collapsed. Loken survived and witnessed the final bombing of Istvaan III. To prove his worth and loyalty to Lord Commander Eidolon - and thusly to his Primarch, Fulgrim - Lucius of the Emperor's Children turned against the Loyalists within the Legion, slaying them personally. In the end, the Loyalists retreated to their last bastion of defense, only a few hundred of their number remaining. Finally, Horus ordered his men to withdraw, and then had the city bombarded into dust for a final time from orbit.

Flight of the Eisenstein
The seventy Loyalists led by Captain Garro 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. The Eisenstein was badly damaged during its escape from Istvaan III; all its astropaths were 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 engines to self-destruct and ejecting them from the ship. Rogal Dorn's Imperial Fists Legion had been becalmed in the Warp with its fleet for some time, and his Navigators sensed the detonation of the Eisenstein's Warp drives. Making an immediate course for the location of the ship's beacon 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 who escaped from Horus' flagship, the Vengeful Spirit, and Garro's dogged insistence convinced the Primarch, and the Phalanx fortress set a course for Terra.

The fate of the Eisenstein survivors is unknown. Sequestered on Luna in a tower belonging to the Silent Sisterhood after arrival in system, Garro, the rest of his Death Guard, Iacton Qruze of the Luna Wolves, and the warriors of the Silent Sisterhood faced one of the Death Guard who had succumbed to the temptations of the Chaos God Nurgle. Afterwards, Garro and Captain Qruze were met by Malcador the Sigillite, First 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. These Space Marines are thought to be the founding members of the Imperial Inquisition and the militant arm of its daemon-hunting Ordo Malleus - the Grey Knights.

The Route to Terra
After the Drop Site Massacre, it became clear that eight of the eighteen Legions had turned to Chaos. Horus openly 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 leadership of the Traitor Legions, supported by elements of the Imperial Army, a portion of the Adeptus Mechanicus, strong Imperial naval forces then still part of the Legions themselves, and the daemon-spawn of Chaos. Their aim soon became clear: Terra, the heart of the Imperium of Man.

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 Guilliman and the Ultramarines on Calth.

Rogal Dorn and Malcador the Sigilite, receiving the few survivors from the Dropsite Massacre became aware of the full implications of their position. Dorn immediately recalled all Imperial forces back to Terra in preparation for Horus' invasion.

Of the Legions still loyal to the Emperor the Space Wolves had just completed the Burning of Prospero, near the Chondax System, where the White Scars were stationed. Without warning, the Alpha Legion's fleet broke from the Warp and engaged the forces of Leman Russ, hammering his smaller force and forcing Russ to resort to hit and run attacks. The Alpha Legion's Primarch Alpharius also attacked the nearby White Scars piecemeal in an attempt to draw the larger Legion into conflict.

The White Scars PrimarchJagahatai Khan wished desperately to aid Russ, yet as the Traitor Legions' ships attacked, he received the order from Rogal Dorn. Khan was to bring his Legion back to Terra, immediately. Dorn also ordered 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, 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 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 Primarch Roboute Guilliman and the remnants of his Ultramarines' 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, co-ordinating each pocket of defence. One such pocket, under Brother-Captain Ventanus, organised a breakout, and retook Calth's defence cannons and laser silos, reaping a great tally of Traitor space vessels. 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 off. Reunited, the Ultramarines received Malcador's orders and set out for Terra, late.

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

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

As the Warmaster was moving for Terra he received an unexpected communication from the recently betrayed Thousand Sons Primarch Magnus the Red. The Space Wolves had driven the Thousand Sons from Prospero. Magnus pledged his allegiance and the allegiance of the Thousand Sons Legion to Horus and the Chaos Gods in retaliation against the Emperor for this betrayal. The Thousand Sons were en route to Terra where they would link up with Horus' forces.

Of the nine remaining Loyalist Space Marines Legions, only the White Scars and Blood Angels were able to join Rogal Dorn and his Imperial Fists in the defense of Terra. Three entire Titan Legions of the Adeptus Mechanicus and close to two million soldiers of the Imperial Army stood alongside them.

The Landing on Terra
The siege began with an orbital bombardment by the Warmaster, the prelude to invasion. Although the Loyalist fleets and defences fought back and the defences on Luna reaped more than a quarter of Horus' fleet they, like the Loyalists soldiers on the surface, were too few, and were mowed down without mercy.

After days of bombardment, the Traitor Space Marines landed on the surface in drop pods and advanced on the two spaceports nearest the Imperial Palace. Five Traitor Legions participated, combining with Traitor forces on the surface. Despite the brave efforts of the Loyalists, the Eternity Wall and the Lion's Gate Spaceports fell within hours. 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' troops landed en masse, and the hulking transports carried thousands of troops each. The transports' immense size made them prime targets for Terra's defence lasers. Although many landing ships were destroyed, many more landed on the surface, disgorging yet more soldiers, tanks and Traitor Titans to add to the besiegers' strength. They met stiff resistance, the Imperial defenders knowing that the survival of their homeworld, their Emperor, and the entirety of the human race rested on their shoulders.

The Siege
The besiegers forced the defenders back to the walls of the Imperial Palace, where thousands died slowing the assault. Angron, now a Daemon prince of the blood god Khorne, came forth 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 had it not been for the Primarch Sanguinius, winged leader of the Blood Angels. The two Primarchs gazed at each other, probably communicating telepathically. Eventually Angron withdrew, telling his forces there would be no surrender.

The siege 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, forces led by Jaghatai Khan, Primarch of the White Scars, 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) Traitor Titan Legion to demolish entire sections of the wall. Despite grievous losses, the Titans gouged open breaches in the Imperial Palace's defenses, which the Traitors flooded through.

Jaghatai Khan decided on a change of plan. Rather than assaulting the almost-invincible flanks of the besiegers' army, Khan redirected his White Scars and the surviving Loyalist Tank Divisions to Lion's Gate Spaceport. At dawn Jaghatai's lightning raid caught the Traitor garrison completely by surprise, and reclaimed the spaceport. The Khan ordered his troops to reactivate the defense lasers 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's unprotected dropships. The Khan's plan worked: the flow of men and machines to the Palace had been cut in half at a single stroke. Inspired by this success, Loyalists tried to seize back the Eternity Wall spaceport, but were driven back without difficulty.

Inside the Palace, the defenders had been forced back to the Eternity Gate, the sole point of entry into the Imperial Palace. The Blood Angels and Imperial Fists tried to hold back the attacking Chaos troops, while the remaining Loyalists made it through the Gate. Soon the mighty Bloodthirster daemon Ka'bandah came forth and bellowed out a challenge to Sanguinius. 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 siege, Sanguinius was cast down by the daemon, pulverising the concrete below upon impact. The Loyalist forces seemed to collectively groan.

Yet the Blood Angels' Primarch was not beaten. 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. The Eternity Gate was closed.

The Endgame
The siege of Terra lasted 55 days. Both sides knew the defeat of the Imperium was near. Sensing this, Horus prepared to teleport to the surface 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, some believe Horus disabled his shields as he experienced one last moment of regret, and some believe it was a personal challenge to the Emperor. Nevertheless, Horus lowered the shields of his flagship The Vengeful Spirit.

The Emperor rose to the challenge, leading his elite Adeptus Custodes, the Primarch Sanguinius, Rogal Dorn, and several companies of Imperial Fists and Blood Angels Veteran Marines in the assault. Horus used his powers to scatter the Emperor's force throughout the massive warship. Each fought a series of battles aboard the corrupted starship, attempting to link up with their comrades and confront Horus.

It was Sanguinius who reached Horus first. The Warmaster attempted to turn the Blood Angel Primarch to Chaos. When Sanguinius refused, Horus attacked. Wounded from his many battles on Terra, Sanguinius was no match for Horus, now at the peak of his daemonic power. Horus strangled the Angel of Baal with ease. An alternate version of this event has Sanguinius cutting a small hole in the Warmaster's armor before he died. And this hole aided in the Emperor's final defeat of Horus.

When the Emperor entered, he saw the corpse of Sanguinius lying at Horus's feet. Horus called the Emperor foolish for refusing the power that the Gods of Chaos offered, and timid for not taming them to his will. If the Emperor would kneel before him, then he would spare his life. The Emperor knew well the trap that had snared Horus. He told him that he was the deluded servant of Chaos, not the master. Snarling, Horus hurled bolts of daemonic lightning at the Emperor, but the Emperor nullified them. The die was cast. Each god-like being knew that the fate of humanity hung in the balance.

The Emperor and Horus engaged one another, battling physically and psychically. Though the Emperor's psychic gifts and martial skills were unequalled, 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 runesword and 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, bursting an eye. After tearing the Emperor's right arm from its socket, Horus raised his father high over his head, and broke his back over his knee.

At that moment, a lone Adeptus Custode entered the bridge. Horus showed him the Emperor's broken form and laughed at the Custode. He roared and charged the Warmaster. He was flayed alive by a glancing psychic blast from Horus. [In previous editions of the tale, an Imperial Fist Terminator attacks Horus; in older versions, the doomed man is an Imperial Guardsman named Ollanious Pious.]

The casual brutality of the act galvanised the Emperor. Realising at last that his favoured son was truly lost to the corruption of Chaos, the Emperor finally gathered his full and awesome power in the Immaterium and unleashed a lance of pure Warp energy that pierced the gloating Horus' psychic defenses. Just before Horus died, he looked his father in the eye, shedding a single tear, begging his father to finish 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 compassion from his mind, the Emperor destroyed Horus utterly, his essence burned from existence.

The destruction of Horus' soul sent a psychic shockwave surging across the Solar System, casting the Chaos daemons back into the Warp, spreading mass panic among the Traitor Legions in seconds. It became clear to the forces of Chaos that their leader had been defeated. A berserk fury 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 turned to bloodbath; thousands upon thousands of Traitor Marines and Titans fell attempting to flee. The ground before the Sanctum Imperialis ran red with the blood of traitors and heretics.

Meanwhile, Rogal Dorn finally found his way to the corrupted starship's bridge, only to discover his fallen brother, Sanguinus, and the Emperor, now on the verge of death, his energy spent. It was then that the Emperor whispered instructions to Dorn, urging the Imperial Fists' Primarch to take him to the Golden Throne. The surviving Loyalists teleported back to the Imperial Dungeons beneath the Imperial Palace. Here Malcador the Sigilite, who had briefly taken the Emperor's place on the Throne, thus keeping the warp-gate beyond it closed, collapsed to dust as he was removed and the Emperor put in his place.

The Emperor spoke his final words to his followers. He urged them to continue the fight to free humanity from the forces of Chaos and 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 in a crippled, decaying body for millennia. The Imperium of Man survived, but would become the bastion of repression and brutality the Emperor had long fought against. It would also offer humanity its best hope for survival in an uncaring and increasingly hostile universe.

For more information on the Endgame see Horus (Warhammer 40,000) and Emperor of Mankind (Warhammer 40,000).

The Aftermath
As the flames of the civil war subsided, Ultramarines Primarch Roboute Guilliman rallied the surviving Loyalists, stretching his Legions thin across the galaxy to buy time for the other Legions to rebuild and rearm. Although some Chaos Legions had fled to the safety of the Eye of Terror, the Iron Warriors Traitor Legion held fast on the worlds they had conquered, and were dislodged only after decades of gruelling warfare. Similarly, the 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 Tsagualsa. Shortly after, Roboute Guilliman slew the Alpha Legion Primarch Alpharius on the planet of Eskrador.

As the last of the Traitor Legions fled to the Eye of Terror, Roboute Guilliman turned his attention to rebuilding the Imperium of Man. Never again would it succumb to 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. The Inquisition was founded around this time (some sources suggest it was formed before the Horus Heresy), and the Grey Knights Space Marines Chapter was founded to combat daemonic incursions as part of the Inquisition's Ordo Malleus. The Council of Terra was succeeded by the High Lords of Terra: the 12 most powerful individuals in the Imperium who were the leaders of its various Adepta, who were to act as the regents of the Imperium.

Of all the changes brought upon the Imperium of Man by Roboute Guilliman after the Horus Heresy, the most pivotal was the Second Founding of the Space Marines. This saw the remaining Loyalist Space Marine Legions broken up into smaller, more flexible 1000-man units known as Chapters. Although Rogal Dorn, Vulkan, and Leman Russ opposed it, Guilliman had the backing of Corax, Jaghatai Khan and the High Lords of Terra, and the measure passed. Never again would one man wield the power of an entire Space Marine Legion.