Black Legion - Fall to Chaos: 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 standard 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 organised human 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 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 after his XVIIth Legion had been humilated by the Ultramarines Legion on the direct orders of the Emperor at the world of Khur.Angered and wounded that the Emperor would not accept his devotion and worship, Lorgar began what became known as the Pilgrimage of Lorgar to discover the truth of divinity in the universe. This quest ultimately culminated in Lorgar turning to the service of the Ruinous Powers of the Warp -- Dark Gods 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 the XVIIth Legion's First Chaplain 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, a former Imperial Army commander, 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 gifted the weapon to the Chaos-corrupted form of Temba who the Warmaster had left behind on the Feral World of Davin sixty Terran 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 Nurglite 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 that 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.Sixty standard years earlier, when the Luna Wolves had brought about the successful Imperial Compliance of Davin, they had entrusted a detachment of the Word Bearers, under the command of its First Captain Kor Phaeron, to shepherd the people of Davin into the light of the Imperial Truth. At the suggestion of First Chaplain Erebus, the XVIth Legion had adopted the native Davinite institution of "Warrior Lodges".Though these lodges had begun as simple fraternities of warriors, their secretive nature handed Lorgar and Erebus the tool they needed to manipulate Horus. Once formally introduced, these Warrior Lodges would infiltrate and eventually corrupt the various Space Marine Legions into turning against the Emperor.After the Warmaster was struck down on Davin's moon, Erebus saw his chance to further the designs of Chaos. The First Chaplain persuaded the Sons of Horus' Warrior Lodge to allow a group of Davinite shamans to heal Horus. In truth, these shamans were secretly a coven of Chaos Cultists known as the  Serpent Lodge that had long been active on Davin at the temple dedicated to the Chaos Gods they called the Delphos. Desperate to save their beloved Primarch, the Warrior Lodge brothers acquiesced to Erebus's suggestions.During the dark rituals that followed within the temple, Horus' spirit was transferred from his body into the Immaterium. There, he bore witness to a nightmare vision of the future. He saw the Imperium of Man as a repressive, violent theocracy, where the Emperor and several of His Primarchs (but not Horus) were worshipped as Gods by the masses. While this vision of the Imperial future granted by the Chaos Gods was a true one, it was ironically an outcome largely created by the Warmaster's own actions. The Dark Gods portrayed themselves as victims of the Emperor's psychic might, and claimed that they had no real interest in the happenings of the material world.Magnus the Red, the sorcerous Primarch of the Thousand Sons Legion, had also travelled into the Warp via sorcery to try and stop Horus from turning to Chaos. Magnus explained that the Warmaster's vision was only one among many possible futures, but one that Horus alone could prevent. Horus, already jealous and resentful of the Emperor, proved all too receptive to the Ruinous Powers' false vision.The Chaos Gods' pact with Horus was simple: "Give us the Emperor and we will give you the galaxy." Driven by his jealousy, desire for status and 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 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 a human extension into the Webway.Refusing to believe that Horus, His most beloved and trusted son, would actually betray Him, the Emperor instead mistakenly perceived the traitor to the Imperium to be Magnus and his Thousand Sons, who had long suffered from a near-debilitating run of mutations because of the instability of Magnus' own genome and were known to have practiced the sorcery that had been expressly outlawed in the Imperium by the Council of Nikaea.The Emperor ordered the Primarch Leman Russ, Magnus' greatest rival, to mobilise his Space Wolves Legion and the witch hunters known as the Sisters of Silence and take Magnus into custody to be returned to Terra to stand trial for violating the Council of Nikaea's prohibitions against the use of psychic powers within the Imperium. While en route to the Thousand Sons Legion's homeworld of Prospero, Horus convinced Russ, who had always been repelled by Magnus' reliance on psychic powers, to launch a full assault on Prospero instead, even though Magnus had been entirely willing to face the Emperor's judgment once he realised he was being manipulated by the entities that called the Immaterium home.