The Chaos Gods, also called the Dark Gods or the Ruinous Powers, are powerful entities who inhabit and control the psychic dimension that underlies all physical reality known as the Immaterium or the Warp in the Warhammer 40k Universe.
They are created and sustained by the emotions and collective desires of every sentient being of the material universe. When an emotion or belief in realspace grows strong enough, it becomes embodied as one of the sentient denizens of the Warp.
The most powerful and most malevolent of these have become the Gods of Chaos. Although they are god-like beings, the Dark Gods are by their nature monomaniacal and completely single-minded since they are formed entirely of a single emotion or concept. The Chaos Gods are dependent upon the emotions of mortal creatures, especially the hordes of humanity, for their power and continued existence.
As a result, the Chaos Gods strive to convert all mortals to their worship and service so that they may ultimately dominate the universe. However, if they were to win such a dark victory, it would likely destroy all of reality.
The Chaos Gods and the forces of Chaos that serve them are the greatest enemies of the Imperium of Man, the Aeldari and most other sentient forms of life in the Milky Way Galaxy.