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 xeno had to fucking die for the crime of not being human and therefore HAWT.

Then Slaanesh appeared before him and he was so much HAWTER then Fulgrim that he dropped down on his knees and started sucking Slaanesh's cock. And some kind of Eviiiiillleeeee fucking clone took over his body and turned him into a giant painting in a dusty old closet where Slaanesh stores his dildoes

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

Oh, and he's the primarch of the Emperor's Children.