Horus - Corruption by Chaos: Shortly after the Luna Wolves Legion and their allies declared victory in the great Ullanor Crusade against the mightiest Ork empire faced by the Imperium until the 41st Millennium, the Emperor offered Horus the honour of renaming his Legion the Sons of Horus in honour of himself and to show his preeminent place among the other Primarchs.Horus refused this honour, characteristically not wishing to be set above his brothers, but was promoted to the newly-created rank of Imperial Warmaster, serving as the new Supreme Commander of the Imperium's millions-strong armies. The Emperor then left the Crusade in Horus' capable hands and returned to Terra to pursue the secret Imperial Webway Project that he hoped would draw the newborn Imperium together in unbreakable bonds.Despite the majestic rank and unparalleled authority bestowed on him, it was said that Horus was not content. The wording of the Emperor's declaration, claiming the glory of Horus' victories as entirely his own, chafed. Although this was the usual rhetoric for such Imperial announcements, Horus saw that while the Emperor remained behind in the Imperial Palace on Terra for reasons that he would not share even with his sons, Horus would be out in the field of battle, winning the Emperor's Imperium for him. It seemed that a deep-rooted resentment and jealousy had begun to simmer in the corners of the Warmaster's mind.As the Imperial Warmaster, Horus took over command of the Great Crusade, and accepted his new duties with earnest dedication. However, there was much dissension in the ranks of the Primarchs and other parties in the Imperium over the Emperor's decision to withdraw from the campaign and return to Terra as well as to reorganise the political administration of the Imperium under the control of a Council of Terra headed by his regent, Malcador the Sigillite.Only a handful of the Primarchs, amongst them a scheming Lorgar, remained steadfast beside the Warmaster during this period of conflict. Horus also disagreed with many of the decrees passed by the newly established Council of Terra, a ruling body of Imperial nobles and bureaucrats, which were intended to shift the burden of taxation and administration onto the newly-conquered Imperial Compliant worlds.Even worse, Horus came to believe in his heart that he was failing his father, and was deeply wounded that the Emperor had revealed to none of the Primarchs, not even his most favoured son, why he had secluded himself upon Terra and the truth behind his secret Imperial Webway Project. These seeds of bitterness, resentment and frustration grew, and would soon bear deadly fruit.It was around this time that Horus, upon the advice and counsel of his greatest friend among his fellow Primarchs, Sanguinius of the Blood Angels, decided to change the name of the Luna Wolves to the Sons of Horus after all. Sanguinius had argued that the Emperor had suggested the honour precisely to show his great confidence in Horus. Bearing such a designation would show the other Primarchs that Horus had earned his right to command them through the Emperor's trust in his judgment and leadership.First Chaplain Erebus of the Word Bearers, the XVIIth Space Marine Legion that was secretly in league with the Forces of Chaos since they had been humiliated by the Emperor for their violations of the Imperial Truth on the world of Khur over 40 Terran years before the start of the Horus Heresy, became a very close confidante and adviser of Horus.The Words Bearers, who originally had worshipped the Emperor as a god and whose Primarch Lorgar had penned the Lectitio Divinitatus before he turned against his father following the events on Khur, had been brutally reprimanded for their constant erection of temples and shrines dedicated to the worship of the God-Emperor on newly conquered worlds. This was a policy that had directly violated the Emperor's atheistic Imperial Truth and had slowed the XVIIth Legion's progress in the Great Crusade to a crawl.In the wake of his humiliation and the reprimand against his Legion on Khur, Lorgar had sought to discover whether there truly were divinities worthy of human worship during the quest that came to be known as the Pilgrimage of Lorgar. During this journey, Lorgar entered the Eye of Terror and came face-to-face with the power of Chaos, a force that he believed was truly divine and worthy of his service and the worship of all Mankind.He brought this faith to his Legion, and soon all the Astartes of the Word Bearers believed that the Chaos Gods were more worthy of their loyalty and worship than the Emperor who had proved himself to be a false god. With the guidance of the Ruinous Powers, Lorgar and the Word Bearers hatched their secret plans over the course of the next four solar decades to convert humanity to the service of the Dark Gods, starting with Horus.After Lorgar placed his agent near Horus, Erebus slowly managed to twist the thinking of Horus against the Emperor and turn half of the Mournival, the Warmaster's most trusted advisers within the XVIth Legion, towards the same path of treachery by exploiting their intense loyalty to their Primarch over that for their far more distant Emperor. This plot reached its climax on the moon of the Feral World of Davin, where Horus was wounded during a struggle against Eugen Temba, the former Planetary Governor of Davin who had rebelled against the Imperium following his corruption by Chaos.An ancient xenos artefact blade sacred to Nurgle known as the Kinebrach Anathame had been stolen by Erebus from the museum of an advanced human civilisation called the Interex during the Sons of Horus' brief sojourn on their world of Xenobia and had been given by him to Temba. Temba had become the mutated servant of Nurgle, the Chaos God of disease and decay, and managed to seriously wound the Warmaster with the corrupted blade. The Apothecaries of the XVIth Legion proved incapable of healing the Warmaster's wounds despite every form of advanced medical technology at their disposal, until it seemed that Horus would surely die.Erebus then convinced the officers of the Sons of Horus' warrior lodge to allow him to take the Warmaster to a secret sect on Davin's moon and use sorcery at this Temple of the Serpent Lodge in order to treat Horus. The creation of a warrior lodge within the Luna Wolves and many other Space Marine Legions based on the lodges used by the savage warriors of Davin had been another of Lorgar and Erebus' ploys to infiltrate and corrupt the Astartes into turning against the Emperor across the decades.However, not all Astartes of the Legions joined the lodges, as many saw them as a direct violation of the Emperor's desires that all Space Marines dedicate themselves to truth and openness with others and among themselves. He also presciently believed that secret cabals created within military organisations almost always led eventually to conspiracies and betrayal.The lodges would prove to be the primary purveyors of corruption in those Legions which turned Traitor in the days immediately preceding the start of the Horus Heresy. Those Astartes within those Legions which lodges which held themselves aloof from those secretive organisations were marked as Imperial Loyalists who would later be betrayed at Istvaan III.The Serpent Lodge on Davin was in truth a Chaos Cult, and using sorcery, which was outlawed by the Emperor, the cultists managed to warp the mind of the Warmaster against the Emperor by playing on the seed of jealousy and resentment that he felt for his father after the Emperor had left the Great Crusade behind to return to Terra. During the dark rituals that followed within the temple, Horus' spirit was transferred from his body into the Immaterium.There, he bore witness to a nightmare vision of the future. He saw the Imperium of Man as a repressive, violent theocracy, where the Emperor and several of his Primarchs (but not Horus) were worshipped as gods by the masses. While this vision of the Imperial future granted by the Chaos Gods was a true one, it was ironically an outcome largely created by the Warmaster's own actions.The Dark Gods portrayed themselves as victims of the Emperor's psychic might, and claimed that they had no real interest in the happenings of the material world. Magnus the Red, the sorcerous Primarch of the Thousand Sons Legion, had also travelled into the Warp via sorcery to try and stop Horus from turning to Chaos. Magnus explained that the Warmaster's vision was only one among many possible futures, but one that Horus alone could prevent.But 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 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 psychic sorcery that had been forbidden several years earlier at the Council of Nikaea. 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 by taking on astral form in the Warp. The message ultimately 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 known as the "Flesh-change" because of the instability of Magnus' own genome and were known to have practiced the sorcery that had been expressly outlawed in the Imperium.The Emperor ordered the Primarch Leman Russ, Magnus' greatest rival, to mobilise his Space Wolves Legion and the witch hunters known as the Sisters of Silence and take Magnus into custody to be returned to Terra to stand trial for violating the Council of Nikaea's prohibitions against the use of psychic powers within the Imperium.While en route to the Thousand Sons Legion's homeworld of Prospero, Horus convinced Russ, who had always been repelled and horrified by Magnus' reliance on psychic powers, to launch a full assault on Prospero instead, even though Magnus had been entirely willing to face the Emperor's judgment once he realised he was being manipulated by the entities that called the Immaterium home. In this way, Horus was successful in convincing Russ to become Magnus executioner instead of his gaoler. The Burning of Prospero was the tragic result.