Chaos Gods
The four gods which rule over the Realm of Chaos in Warhammer Fantasy Battles and the Warp in Warhammer 40,000. They love nothing so much as dicking with each other, except perhaps dicking with their mortal followers. Each one is formed by the emotions clumped together in the Warp/Realm of Chaos. They are:
Khorne: BLOOD FOR THE BLOOD GOD! SKULLS FOR THE SKULL THRONE! MILK FOR THE KHORNE FLAKES! Oh, sorry. In case it wasn't obvious, Khorne is the god of battle, martial honor, and oh yeah, BLOOD! Although primarily formed from hate and rage, bravery and honor are also thrown in the mix.
Nurgle: The god of filth, pestilence, decay, and generally being a cool dude. Formed from despair and fear of death, acceptance and stoicism are also included. Nurgle is also notable for being the only Chaos god that cares for his followers whatsoever, bordering on love (in fact, in 40k, he loves the Eldar goddess Isha so much that he chained her up and force feeds her his new diseases... yeah, love sucks sometimes).
Slaanesh: The god of pain, pleasure, and perfection... or in other words, a god of emotions formed from emotions. Formed mainly from hedonism and excess, love and creativity are also attributes of Slaanesh. In 40k, the Eldar created him/her by having so many damn orgies they tore a hole in space. I shit thee not.
Tzeentch: The god of progress, knowledge, and Just as planned. Formed by hope, ambition, and the desire for change.
The Other Ones
Older editions of Warhammer Fantasy mention several lesser Chaos gods. These only appear in WFB (and even then, only sparingly in later editions), with some (the Horned Rat and Hashut in particular) generally considered to be separate from the "main" Chaos pantheon. According to Lexicanum they are:
Be'Lakor: A fallen Chaos champion, who was apparently so crazy powerful that becoming a daemon prince was a demotion. Somehow still able to grant his followers unique magic powers.
Hashut: Father of darkness and god of the Chaos Dwarves. And if his followers are any indication, god of penis-compensating hats.
The Horned Rat: Patron of the Skaven, and god of...well, rats. Also disease, mutations, etc., but mostly rats. The Horned Rat once appeared in material form; he's the only Chaos god to do so. Of course, he just started eating a ton of the Skaven present, but still.
Malal: Renegade Chaos god. Interbutt lore states that he is the god of fractiousness and dissent, which, given the company he keeps, makes him a tad redundant.
Zuvassin: A minor god who just likes to see shit fall apart, specifically nonphysical stuff like schemes and lives. He and Malal are the co-chairs and so far only members of the Useless-Ass Gods Club (Necoho would join the club, but he doesn't believe it exists).
Necoho: The god of atheism. I wish I was making that up.
