Fulgrim

Fulgrim was the HAWTEST goddamn ass hole in the galaxy. He was so hot, that his residual HAWTNESS made the rest of humanity the HAWTEST race in the galaxy. Therefore, all the ugly xenos had to fucking die for the crime of not being human and therefore HAWT.

Horus Heresy

During the heresy, Horus was like "join us", then Fulgrim was like "sure." After he rebelled he killed his best friend. Then Slaanesh appeared before him and he was so much HAWTER than Fulgrim that he dropped down on his knees and started sucking Slaanesh's cock.

Then some kind of Eviiiiillleeeee fucking clone took over his body and turned him into a giant painting in a shitty old closet where Slaanesh stores his dildos

Theeeeen he turned into a snake. Yeah, I don't quite understand that part either.

By the way he's the primarch of the Emperor's Children.

Oh there's a bit more then just that... Fulgrim is like fucking retarded... he picks up a sword which just whispers 24/7 "FUCK EVERYONE TO DEATH!!!" in his head all the time and Fulgrim is like "This is a completely normal sword that is not telling me to fuck all you bitches to death." and everyone believes him. Even after Elrad himself says the sword is fucking insane and is trying to make him like fuck himself and the whole galaxy to death, Fulgrim is like "NAH-Ah YOUR MOMA'S INSANE, BITCH!" And then right when the Heresy kicks off and Slaanesh starts fucking about in his body, Fulgrim is all like "I'M SOO FUCKING STUPID!!! AND I'M STUCK IN THIS PAINTING BECAUSE I'M A RETARD!".

TL-DR: Fulgrim's sword fucked him in the ass.