[[Imagefile:The_gorgon_and_the_phoenix_by_kokoda39-daooq8r.jpg|thumb|right|250px|Fulgrim battles [[Ferrus Manus]] ]]It is now believed that this daemonsword the [[Silver Blade of Laer]] began to exert a strong chaotic influence over Fulgrim, and that the forces he had deployed against the Laer may have become tainted by the [[Chaos]] power [[Slaanesh]]. It was in this perilous spiritual condition that Fulgrim found himself at the centre of the emerging [[Horus Heresy]].{{Fn|1}}

Fulgrim himself met with the renowned [[Farseer]] [[Eldrad Ulthran]] of [[Ulthwé]] on the [[Maiden World]] Tarsus, in which ; the Farseer attempted to warn Fulgrim of Horus's grievous wounding at the hands of [[Eugen Temba]] and his [[Anathame]], and how he was slowly beginning to turn to Chaos as he recuperated.{{Fn|2f}} Fulgrim reacted with outrage at the Farseer's accusations due to his close friendship with Horus, second only to his bond with [[Ferrus Manus]]. This was spurred on by the influence of Fulgrim's [[Laeran]] blade, which was infused with a [[Slaanesh]]i entity. Fulgrim furiously attacked Eldrad alongside his captains and the [[Phoenix Guard]], destroying both [[Khiraen Goldhelm]], an ancient and revered [[Wraithlord]], and an [[Avatar]] of [[Kaela Mensha Khaine|Khaine]] in the process, forcing Eldrad and his beleaguered forces to withdraw. This victory led to Fulgrim's subsequent destruction of various [[Eldar]] maiden worlds using [[virus bombs]] as a means to end the supposed treachery of the Eldar.{{Fn|2g}}

Whilst the exact timing and placement of it varies between versions of the story, it is clear that Fulgrim soon met Horus in person, demanding a personal account of his actions. Instead, Horus was able to sway Fulgrim to his cause. Fulgrim's respect for Horus allowed Chaos to find its way into Fulgrim's heart, destroying Fulgrim's loyalty to Terra, and replacing it with a burning desire to destroy the man who held [[human]]ity back from the perfection Fulgrim desired.{{Fn|1}}{{fn|2i}} Fulgrim was immediately sent to meet with [[Ferrus Manus]], Primarch of the [[Iron Hands]]. Great bonds of friendship and brotherhood existed between them, and Fulgrim felt that he could convince Ferrus of the righteousness of Horus's cause. He was wrong, however, and their meeting did not go well, concluding in violence. When next the brothers met, it would be as enemies.{{Fn|2j}} Their path chosen, the chaotic rot spread quickly, from Fulgrim to his lieutenants, the Lord Commanders of the [[Space Marine Legion|Legion]], then to company and squad leaders, and finally all but a bare handful of Marines followed Slaanesh rather than the Emperor. Perfection became perfect hedonism. Their path chosen, the corrupting influence of Chaos spread rapidly through Fulgrim and his sons, until almost the entire Emperor's Children Legion had turned to Slaanesh. During the Drop Site Massacre on Isstvan V, Fulgrim and his captains betrayed the loyalist members of the Legion, slaughtering their former brothers. The pursuit of perfection gave way to indulgent cruelty as the Legion fell fully under the sway of Slaanesh.{{fn|2m}} Fulgrim's mental state quickly deteriorated, he began talking to a painting by [[Serena D'Angelus]]{{fn|2h}} and killed the sculptor [[Ostian Delafour]] after he inadvertently slighted the Primarchprimarch.{{Fn|2k}}

Fulgrim soon joined forces with [[Perturabo]], Primarch of the [[Iron Warriors]], on the world of [[Hydra Cordatus]]. Promising the Lord of Iron perfection, the two embarked on a quest to tip the balance of the rebellion in Horus's favor by journeying into the [[Eye of Terror]] and retrieving a weapon known as the '''Angel Exterminatus'''.{{Fn|5b}} Shortly before they were to leave, Fulgrim was hit by a direct headshot from the vengeful ''[[Sisypheum]]'' crew member [[Nykona Sharrowkyn]] of the [[Raven Guard]].{{Fn|5c}} Fulgrim survived the wound, however, thanks to the efforts of [[Fabius Bile]].{{Fn|5d}} After a brief boarding battle with the ''Sisypheum'' that saw the ship escape yet again{{Fn|5e}}, the two Primarchs primarchs arrived in the Eye and quickly landed on the [[Crone Worldcrone world]] of [[Iydris]].{{Fn|5f}} Entering an ancient [[Eldar]] citadel known as the ''Amon ny'shak Kaelis'', the Emperor's Children and Iron Warriors [[Battle of Iydris|had to contend]] with an army of [[Wraithguard]] and [[Wraithlord]]s that had awoken from their slumber.{{Fn|5g}} At the height of the vicious battle, Fulgrim attempted to betray his brother and slip away, but was pursued by Perturabo.{{Fn|5h}} Both soon arrived in a massive spherical chamber, and Fulgrim revealed to his brother that there was no Angel Exterminatus, but rather he was destined to become such a weapon on this world. Furious, Perturabo attempted to attack Fulgrim, but was drained of his power when Fulgrim activated the ''Maugetar'' stone that Perturabo had previously received under the guise of a gift.{{Fn|5i}} Now with the Iron Warriors Primarch's power in hand, Fulgrim revealed the purpose of his plan: to achieve ''Apotheosis'', better known as [[Daemon Prince|Daemonhooddaemonhood]].{{Fn|5j}}

However, just as Fulgrim was about to complete his ritual and achieve Daemonhooddaemonhood, Perturabo gathered enough strength to charge at his brother but ; they were interrupted by an ambush of [[Salamanders]], [[Iron Hands]], and [[Raven Guard]] who had been stalking the traitor fleet in search of vengeance since the [[Drop Site Massacre]]. One of the loyalist Astartes during the battle shattered the ''Maugetar stone'', freeing some of Perturabo's energy and giving him the ability to strike down Fulgrim with his [[Thunder Hammerthunder hammer]].{{Fn|5k}} However, Perturabo only managed to destroy the Primarchs primarch's mortal skin and body, as Fulgrim was reborn as a Daemon Prince daemon prince of [[Slaanesh]]. Now a massive but elegant serpentine creature, Fulgrim told his brother that they would meet again , then vanished in a burst of [[Warp]] energy along with his Emperor's Children.{{Fn|5l}} Fulgrim next appeared answering [[Horus]]'s summons in the [[Battle of Dwell]], where he was tasked to lead with leading the Emperor's Children to the [[Halikarnaxes Stars]]. Before he could depart, Fulgrim survived another Iron Hands assassination attempt, this time by [[Shadrak Meduson]] and a trio of [[Fire Raptor]]s. Fulgrim displayed his potent new abilities in the fight, destroying one of the loyalist aircraft with his psychic powers. Afterwards Fulgrim was tasked by Horus to secretly corrupt the [[Knight House]] of [[House Devine|Devine]] in preparations for the [[Battle of Molech]].{{Fn|9}} Following these tasks, Fulgrim largely vanished as he became more concerned with the matters of the [[Warp]] and his Legion disintegrated into various warbands. However, Horus and [[Eidolon]] were both confident that he would ultimately return when they reached [[Terra]].{{Fn|10}} In 012.M31, his followers summoned Fulgrim during the [[Pale Stars campaign (Horus Heresy)|Pale Stars campaign]] so that he could amuse himself in witnessing horribly mutated [[Inductii]] battle [[Salamanders]]. After taking some enjoyment in personally killing the local Salamanders commander, [[Brant Hesioth]], Fulgrim vanished in a supernova-like explosion.{{Fn|28a}}