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40k: Everything with emotions has an effect on something called the Warp. A single person feeling a bit angry isn't going to do much, but if enough people feel angry the distrubances in the Warp build up. Eventually, this build up becomes a god which feeds on that emotion. There are four big chaos gods and they each feed on a lot of different things.
Tzeentch, god of Change and Sorcery: Feeds on hope, deceit, scheming.
Nurgle, god of Stasis and Disease: Feeds on despair and the ways people deal with fear of death, such as familial love.
Khorne, God of War (aka, the blood god or the skull god): Feeds on hate, rage, blood-lust. That kind of thing.
Slaneesh, Prince of Excess: Feeds on desires, hedonism, sensualism. All about intensity and perfection.