The Great Game is the name given by the denizens of the Immaterium to the constant, unending struggle between the major Chaos Gods for dominance within the Realm of Chaos and ultimate control of realspace. The Empyrean is not only the home of the Dark Gods but it also serves as their battlefield and playground. No Chaos God can ever truly win the conflict, for if the other Ruinous Powers were obliterated, the Warp would be returned to its primordial state of placid serenity and Chaos as a primal force of the universe would cease to exist. The Great Game is the primary concern of the Chaos Gods; they are far more interested in pursuing those stratagems and tactics that will win victory over their fellows than with what occurs in the material realm.
However, on rare occasions, the Chaos Gods will act in temporary unison so as to thwart a common threat -- such as the attempt by the Emperor of Mankind to extend the control of Order over the galaxy during the Great Crusade -- or to take advantage of a specific set of circumstances that will benefit them as a whole. In the early 31st Millennium the Horus Heresy presented one such period, when the Chaos Gods acted in concert to bring about Horus and the Traitor Legions' betrayal of the Emperor so that they might forever put an end to the threat that human unity and the institution of the Imperial Truth presented to the continued existence of Chaos.
The Great Game of the Chaos Gods is also played out amongst their daemonic and mortal followers in the Realm of Chaos and in the Materium. The Greater Daemons, Daemon Princes, Daemon Primarchs and the more powerful Chaos Lords and Chaos Sorcerers all participate in the Great Game on behalf of their divine patrons, for they know that success against their Chaotic rivals will grant them ever greater levels of power and favour.
- Codex: Chaos Daemons (4th Edition)