They are being lead by the Dark Gods, often mistakenly referred to as "Presidents of the U.S." (before Obama, of course).
Notable Chaos ChampionsEdit
Horus was the son of the Emperor, before he went emo and joined Chaos. He managed to shake the Empire to it's foundations during the Horus Heresy, in which he was killed in the final battle while the Emperor was mortally wounded from their battle. Little ungrateful bitch.
The Upreaper is the most gruesome of all Chaos Champions as of yet. He specializes in timeloops, repeating time over and over again to drive his enemies insane. He is in fact so powerful that he has been rumored to gain entry to the rank of the Dark Gods, and has been trying to get his pal Khorne to pull strings and let him join the top rank in the Chaos hierarchy.
If he does become a Dark God and gets his own armada of heretic monsters - then Emperor bless us all.