Chaos, also known to its servants as the Primordial Truth or the Primordial Annihilator, and to the Imperium of Man as the Archenemy, is the universal and usually malign metaphysical force embodied by the malevolent intelligent entities comprised of psychic energy that live in the Immaterium.
The entities who embody Chaos are mostly Daemons, but the term also encompasses those mortals who have thrown in their lot with Chaos, ranging from simple peasants and/or manufactorum labourers who serve as Chaos Cultists, to Traitoris Militarum, planetary militia, Imperial nobles, planetary governors, and even the mighty Chaos Space Marines and Hereteks, Traitor Titan Legions and Chaos Knights of the Dark Mechanicum.
Chaos is also itself the turbulent psychic energy that comprises the alternate dimension known as the Immaterium. It gives shape to the nightmare domains of the individual Chaos Gods that are collectively called the Realm of Chaos by savants of the Inquisition. Chaos is almost synonymous with the Warp — the two concepts are inseparable, for Chaos is the limitless ocean of spiritual, psychic and emotional energy that defines the Immaterium and underlies the 4-dimensional material universe of space-time.
Chaos is a great and raw force of change and power, and is both physically and spiritually corrupting, though it is not in itself necessarily “evil.” While its adherents and servants act in ways that are often malevolent, some devotees of Chaos are more dedicated to the service of freedom and change than of the pure selfishness that Humanity and most other intelligent species define as nefarious.