Fulgrim fell from grace on a xenos planet that was corrupted by Slaanesh. There, during the slaughter of the native xenos race, he found a temple dedicated to Slaanesh. The Imperium, ignorant of Chaos at this time, did not realize the significance of such a find. The expedition led by Fulgrim began to be corrupted by the temple's influence. Fulgrim found a sword with an amythest stone in the pommel. Once he took said blade, it began whispering in his mind and corrupting his soul. Eventually, he succumbed to the whispers of the daemon within the blade and gave himself to Chaos. On Istvaan V, during the massacre that left the Salamanders, Raven Guard and Iron Hands Legions crippled, Fulgrim killed his long-time friend and Primarch, Ferrus Manus in single combat. Realizing what he had done, Fulgrim began to repent, but it was too late. He thought of suicide, but the blade's daemon taunted him and told him that it would give him oblivion if he wished it. Foolishly, Fulgrim relented not realizing the danger. The daemon finally took possession of Fulgrim, keeping his mind subjugated, fully aware of what the daemon was doing, but being powerless to stop it. His own mind went mad with that realization, locked forever within his own shell as the daemon continued on as if it were Fulgrim. The daemon revealed itself only to Horus, and Horus was horrified at the knowledge, but kept the daemon's secret, plotting to kill the daemon and free Fulgrim at a later date, once his plans of taking the Empire as his own were done.
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