In Games Workshop's Warhammer Fantasy and Warhammer 40,000 fictional universe, Chaos refers to the often stereotypically malevolent entities which live in a parallel universe, known as the Warp in Warhammer 40,000 and as the Realm of Chaos in Warhammer Fantasy. The term can refer to these warp entities and their influence, the servants and worship of these entities, or even the parallel universe in which these entities are supposed to reside. The most powerful of these warp entities are those known as the Chaos Gods, also sometimes referred to as the Dark Gods, Ruinous Powers, or the Powers of Chaos. Similarities exist between the Warhammer idea of Chaos and the concept of Chaos from Michael Moorcock's Elric saga, which also influenced D&D's alignment system. Further similarities can be see with the godlike extradimensional Great Old Ones of horror writer H. P. Lovecraft's stories.
The concept of Chaos Gods has been a more or less integral part of both Warhammer universes ever since they were first conceived. The Chaos Gods in Warhammer are essentially deities worshiped and feared by various groups and that is what makes these groups followers of Chaos. In this idea there is evidently a strong influence from the British fantasy writer Michael Moorcock. Many different Chaos Gods were named in the various early miniature catalogues released by Citadel in the early eighties. But it was never clearly explained what the fictional pantheon looked like. The idea of "Four Great Powers of Chaos", i.e. Chaos Gods, was first introduced in the two Realm of Chaos sourcebooks released 1988 and 1990 respectively. To date these remain the original and amongst the most detailed pieces of work published by Games Workshop regarding Chaos. The Black Library "artbooks" of the Liber Chaotica series (released from 2001–2006) and Black Industries' Warhammer Fantasy Roleplay supplement, the Tome of Corruption: Secrets from the Realm of Chaos (published 2006) contain as much detail as the original Realms of Chaos sourcebooks (and, in the case of the Liber Chaotica, taken directly from the original books), but with the various additions and changes to the Chaos imagery that GW has introduced over the years - although these latter two books focus mainly upon Chaos as perceived through the Warhammer Fantasy imagery.
Both game worlds depict fictional settings out of spiritual balance, where emotions and (even more importantly) the departed "souls" of any creature of a psychic nature in the "physical universe" are drawn together, like-to-like, within the "meta-physical universe" that is the "the Warp" or "the Realm of Chaos" (40K and Fantasy respectively), merging into great psychic "storms" or "vortices". As is the case with the "Big Four" Chaos Gods, these "vortices" sometimes gain consciousness, self-awareness and even personality of a sort in the metaphysical (or psychic) context of the Warp. However, this consciousness and/or personality are dominated by the paradigm of the emotions that form the vortex they exude from - so a vortex of anger, rage and of souls and soul fragments that are saturated with feelings of anger and bloodlust (perhaps because they are the souls of dead serial killers or berserk warriors for instance) would be an entirely and eternally furious and violent consciousness / personality.
In the two game settings, Chaos represents the classic theme of Man vs. Himself, with the fantasy twist of giving these psychological struggles independent personalities, drives, influences and names. In a sense, the Chaos Gods in the Warhammer and Warhammer 40K imagery are personifications of specific elements of the "Id". In both settings, Chaos is seen as an insidious and seductive force, a mixture between a "psychic radiation" that can physically and/or psychologically warp and mutate anything it comes into contact with, and also a profound source of "spiritual" temptation that is, in time, capable of contaminating anyone and anything.
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