Fulgrim
From Warhammer 40k
Fulgrim fell from grace on a xenos planet Laeran 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 the blade, it began whispering in his mind and corrupting his soul thinking it was only his own mind talking to him he began listening. Eventually, eventually he found these to be the whispers of the daemon within the blade and after a lot of persuading from his brother Horus, 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 brother 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 loyal, human Fulgrim, as opposed to the masquerading daemon, could only be seen in the portrait of Fulgrim by Serena D'Angelus, which was tranformed from the original (a horrific portrait of the deamonic entity) into a masterpiece. 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.
Later after the Heresy, Fulgrim is later seen fighting Roboute Guilliman, Primarch of the Ultramarines Legion. During their great duel Fulgrim was crafty and slit Guilliman's throat with a poisened blade. Guilliman was recovered and put in a statis field, while Fulgrim retreated into the warp.
