Warhammer 40,000
Parallel to realspace, where mortal creatures live, is the Warp, a realm of pure psychic energy. This realm is influenced by the thoughts, emotions, and urges of all sentient beings. The psychic energies sometimes coalesce into sentient entities. The most powerful of these entities are the four Chaos Gods. They are served by a host of daemons and constantly seek to expand their influence over humanity. Though they are often at odds with each other, their common goal is to crush the worship of the Emperor and convert all of humanity to their cult.
Chaos is a malevolent force that twists the bodies and minds of mortals exposed to it. Followers of Chaos are often lunatics that sport hideous mutations. Many humans worship the Chaos Gods in the hopes of alleviating their misery, advancing their ambitions, or gaining access to magical power, perhaps not understanding the terrible price they will ultimately pay. By far the people most vulnerable to Chaos influence are those with psychic ability. Unless they are carefully trained to resist daemonic possession, psykers can become living conduits by which daemons can enter the mortal world and wreak havoc. Chaos is the root of much of the universe's problems.
There are no major "good" gods in Warhammer 40,000 (unless one counts the Emperor of Mankind, who is not native to the Warp). Positive human feelings like love, altruism, bravery, and invention are perverted by the Chaos Gods. For instance, Nurgle might represent love and mercy, as he genuinely adores his worshippers and considers his plagues to be blessings. This is a deliberate contrivance in order to make the setting as dark and pessimistic as possible.
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