Chaos - Horus Heresy: The primarchs had not escaped their brush with Chaos entirely untouched. As the Great Crusade wore on their dreams became disturbed by the insidious whispers of the Ruinous Powers. Each primarch's character was severely tested by the unspoken urgings, each of them thinking that they alone bore such flaws.The future promise of power became a temptation for some; in others their pride or martial prowess opened a path of spiritual corruption. Little by little their all-too-Human flaws deepened into obsessions. Fully half of the eighteen primarchs eventually failed the test and were seduced by the Ruinous Powers, their corruption occurring in ways so subtle that they never even suspected that their own loyalties were changing until it was too late.Horus Lupercal, the greatest primarch of them all, was utterly convinced of the virtue of the martial ideals for which he fought. He enjoyed the Emperor's greatest trust and the admiration of the other primarchs, gaining the coveted title of Imperial Warmaster in the wake of the Ullanor Crusade of 000.M30.He had stood at the Emperor's side from the earliest days of the Great Crusade and through the many long years that followed. Legends say that they fought back-to-back at the Siege of Reillis and the Emperor saved Horus' life. At the Battle of Gorro, Horus is said to have repaid the debt by hacking the arm from a frenzied Ork warboss that was intent on choking out the Emperor's life.As the Great Crusade advanced the Emperor eventually returned to Terra to administer His rapidly growing domain. He entrusted Horus with leading the crusade along the Eastern Fringes, little realising that by doing so He was planting the seeds of His own betrayal.As the Great Crusade pushed outward in the final solar decades of the 30th Millennium Horus had begun to perceive the Emperor's actions as craven and dishonourable. All too often compromises were made that he felt were weak and unworthy of the Master of Mankind while wanton destruction was unleashed on other worlds on the slimmest pretext. The Emperor abandoning the crusade forces after the Triumph of Ullanor in favour of returning to the administrative centre of the now-vast Imperium in pursuit of some secret agenda confirmed Horus' worst fears. The coming age of enlightenment and understanding preached by the Emperor as the Imperial Truth was nothing but a mask for His own pusillanimous greed.Surely the warriors that had fought so hard to conquer deserved some say in how their conquests were ruled? What kind of man seeks to become sole ruler of the galaxy? So, perhaps, the worms of doubt began to squirm in the brilliant mind of Horus. Perhaps he even foresaw a time when the Emperor would have to be rid of him -- the mighty warrior with no worthy opponents left to fight.Following the events on the world of Davin when he was healed of a mortal wound incurred against the forces of Chaos by Chaos Cultists in service to the Temple of the Serpent Lodge and sealed a pact with the Chaos Gods to save his own life from a Chaos toxin, Horus began to quietly lay plans for rebellion. He subtly sounded out some of the other primarchs and began the slow process of gathering his far-flung forces. Horus' allied Space Marine Legions were gradually seduced by their sense of pride and loyalty to their primarchs into serving their own gene-fathers and the Warmaster above the Emperor.Initially, the Space Marines had little idea that they were being led astray. The taint spread slowly and subtly, and only later when they reached the point of outright rebellion did their veneer of reason fall away to reveal that Chaos had invaded their hearts. In the midst of Horus' preparations, the Imperial Commander of Isstvan III declared the whole of the Isstvan System an independent principality. The Emperor, ignorant of Horus' plans, ordered him to move to pacify the rebellious system.The Warmaster was unwilling to be drawn into a planetary campaign just as his schemes were coming to fruition and chose instead to virus bomb Isstvan III from orbit in what became known as the "Isstvan III Atrocity". Twelve billion souls died in a matter of solar minutes, their rapidly rotting carcasses consumed by a firestorm that enveloped the entire planet for solar days afterwards.During the bombardment a group of Loyalist Space Marines discovered signs of the corruption and treachery in Horus' followers. They succeeded in seizing control of the frigate Eisenstein and fled into Warpspace to warn the Emperor. Horus withdrew to Isstvan V to marshal his forces for the confrontation to come.The Emperor hesitated, shocked by the betrayal of his Warmaster and unable to believe that his best friend, gene-son and general was really marshalling forces against Him. However, even as He learned of the betrayal, dissent and rebellion spread throughout the Imperial Army.Meanwhile, on Mars those who had denounced the Emperor as a False Omnissiah saw that their time had come. The Mechanicum's own fabricator-general, the most powerful magos of Mars, unleashed ancient, forbidden weapons on the surface of the Red Planet as Tech-priests and the Hereteks of the newborn Dark Mechanicum fought for dominance in what became known as the "Schism of Mars".The fragile Imperium tore itself apart as recently-conquered star systems declared independence, and newly-appointed planetary leaders seized their chance to declare for the rebellious Warmaster. Confusion reigned and at first many failed to recognise the resurgence or involvement of the Ruinous Powers, seeing the great conflict as a purely political one between Horus and the Emperor.The rot of Chaos spread quickly, passing from the Warmaster's forces to their allies and even their enemies with shocking speed. Loyalists and Traitors clashed on hundreds of worlds across the galaxy. After an almost fatal delay the Emperor ordered seven full Space Marine Legions to destroy Horus and his rebel forces at Isstvan V. He had recognised finally that the rebellion could only be brought to an end by eliminating its figurehead and inspiration: the Warmaster Horus. More precious solar months were lost organising and mobilising the forces to reach the other side of the galaxy.Horus was not idle in the intervening time and consolidated his hold on hundreds of star systems by declaring himself their new emperor. Wherever Horus' banners were raised the corrupting influence of Chaos quickly followed.The Emperor's retributive strike, now known as the Drop Site Massacre of Isstvan V, proved to be a disaster. Of the seven Legiones Astartes dispatched to destroy Horus, four of them had secretly already been turned against the Emperor by Horus and helped massacre the other Loyalists instead. The Warmaster now controlled nine Space Marine Legions and had all but eliminated three of the Loyalist Legions. Thus, the Horus Heresy began in earnest.The fighting continued for seven more Terran years, and shook the nascent Imperium to its foundations. The Space Marine Legions, the Imperial Army, the Mechanicum, the entirety of Mankind's dominion turned against itself, and ripped itself asunder. The Loyalists eventually began to prevail over the Traitor forces, but Horus knew there was still time to make a decisive strike and win the war.Horus struck directly for his enemy's heart and attacked Terra with the full force of his Space Marine Traitor Legions. The Emperor was caught unawares by the Warmaster's audacious move, and was cut off and besieged inside the Imperial Palace with a bare handful of Loyalists to defend Him.However, the Palace defences proved formidable and the subject people of Old Earth rallied to protect their Emperor, so Horus was denied the quick victory he had hoped for. Bitter fighting marked every phase of the Siege of Terra as it dragged on for over a solar month. Eventually, the mighty Titan war engines of the Legio Mortis breached the towering outer walls and the hordes of Chaos poured through into the Inner Palace.The deeds recorded in the siege of the Imperial Palace would fill a library in their own right, and do so in some parts of the galaxy, but myth and legend has obliterated any hope of knowing the real truth of events. Loyalists hold that at the eleventh hour, the Emperor perceived a weakness in the Void Shields protecting Horus' orbiting battleship, the Vengeful Spirit.According to their holy books the Emperor, along with His two remaining loyal primarchs on Terra, Sanguinius and Rogal Dorn, teleported aboard the battleship to confront Horus. Other sources strongly imply that the Warmaster deliberately lowered his shields to permit the Emperor to come aboard, perhaps in an effort to bring an end to the bloodshed.A battle was certainly fought, and by all accounts Horus slew his brother primarch Sanguinius before himself falling prey to the Emperor. The Emperor was mortally wounded in the battle with Horus, and His physical body all but destroyed. Rogal Dorn is said to have retrieved the Emperor's body and returned it to Old Earth. There He was interred within the Golden Throne where He has remained, unspeaking and unmoving, for ten thousand standard years. Pious followers of the Imperial Creed believe that the spirit of the Emperor still resides within the corpse on the Golden Throne, but many doubtful souls have risked charges of heresy by questioning that supposition.The Warmaster's body was retrieved by his own Legionaries among the Sons of Horus and they fled Terra shortly afterwards. Without the key figure of Horus to hold his followers together the rebellion began to fall apart. Recent converts to the Warmaster's cause switched their loyalties back to the Emperor.The rebel Space Marine Legions turned against one another, servants now of fractious Ruinous Powers rather than any overall strategy. As the Loyalists rallied, Traitor forces were crushed on world after world and the corrupted Space Marine Legions, the "Traitor Legions" as they had become known, were eventually pushed out of Imperial space into the Eye of Terror during the campaign known as the Great Scouring.Across the Imperium, the cult of the God-Emperor was born. In the following centuries it would evolve into the Adeptus Ministorum and the puritanical cult of the Imperial Creed by the dawn of the 32nd Millennium. The Imperium itself evolved into an entity built on the foundations of fear and betrayal laid in the terrible civil war that became known as the "Horus Heresy".A hundred centuries later Horus' actions are painted in the blackest terms and his turning against the Emperor an unthinkable act of villainy. However, outside Imperial space some remember Horus differently, as a proud warrior who was unafraid to stand against the machinations of the Emperor and whose vision for Humanity extended beyond autocratic rulership of Earth.Chaos also won a victory in the Horus Heresy, for the Emperor was crippled by His battle with Horus and had to be interred within the Golden Throne's life support systems, no longer truly alive or dead. This sentenced the Imperium of Man to a long, slow period of decay over the next ten thousand standard years.