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 diminish the hold of Chaos 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 and the Imperial Webway Project presented to the continued existence of Chaos.
Such interest in mortal affairs is fleeting, and as soon as their objective is achieved, the gods resume their Great Game. Sometimes treaties will be broken even before their mutual goals are met, with one god or another, or all four, overstepping the bounds of their agreement and attempting to usurp their rivals. Once again the Realm of Chaos will thunder to the march of the daemonic legions, and their age-old feuds will spill over into the domains of realspace.
With the ebb and flow of psychic energy within the Warp, the power of a Chaos God expands and contracts, and their realm will shift accordingly. For long periods, one god may dominate the others, fed by its own success and leeching its foes’ energy for its own growth.
Eventually, the other gods will ally against the dominant force, and through combined efforts reduce it in power until another of their number rises to prominence. This pattern is played out again and again through eternity. It seems unlikely that any Chaos God could ever truly be victorious, and it is unthinkable what might happen should such an event ever occur, but it certainly is not for lack of trying.
When the gods do battle, the Immaterium shakes and Warp Storms rage across the galaxy. Within the Realm of Chaos, hordes of daemons are sent forth to do their creators' biddings, and the lands of the gods strain and heave at each other in physical assault.
Possessed of personality and intelligence, the daemons of a Chaos God aspire to draw favour from their master, and often launch their own attacks into the domains of rival daemons. The armies of the gods pour from one territory to another in a ceaseless frenzy of invasion and defence.
As intrigue, feints and lures lead forces into traps, elsewhere pacts are forged, and opposing sides join forces mid-battle as a common cause creates a temporary amnesty between rivals. It is never long, however, before the merest possibility of advantage arises, and the brief cessation of open warfare between two parties is readily abandoned so the daemons might once more fall upon one another.
Vast swathes of the Immaterium are in a constant state of flux, every moment a new territory won or lost. When an invading army emerges victorious, they immediately set to work on turning the ravaged battlefield into part of their god's realm, moulding the raw entropy of that section of the Immaterium into whatever form best pleases their master.
The Divine Hierarchy
There exists a hierarchy of sorts within the ranks of the Ruinous Powers, though it ebbs and flows according to the vagaries of the Great Game. Currently, Khorne is held as the mightiest of all, for the practice of murder, war and blood sacrifice stretches to the dark beginnings of the universe.
Though Khorne sees the use of sorcery as the refuge of cowards, his closest rival, Tzeentch, thrives on the raw stuff of Chaos and uses it to influence a million times a million plots, his devious mind always a step ahead of his opponents.
Where Tzeentch would see hopes thrive and fortunes change, Nurgle, the Father of Plagues, revels in despair and hopelessness. In times of galactic pandemic, Nurgle's power can eclipse even that of his brothers in darkness.
Last in the pantheon is Slaanesh, who knows well how to play on the obsessions of his rivals. Khorne's single-minded bloodlust, Nurgle's quest to infect every living thing, and Tzeentch's compulsion to dabble in the fates of mortals -- all are obsessions which the Lord of Excess can turn to his will with a whispered promise.
While the Chaos Gods are all enemies in the Great Game, each bears a special enmity for one of their brothers in particular. Khorne most despises Slaanesh, whose earthly decadence and sensual lusts are at odds with the Blood God's martial pride and desire for indiscriminate slaughter; the Dark Prince finds Khorne's artless brutality dull, and takes a perverse delight in agitating him.
Similarly, Tzeentch's desire to foster the corrupt ambitions and hopes of mortals is at odds with Nurgle's spreading of despair, and so a special rivalry exists between the two.
- Codex: Chaos Daemons (4th Edition), pp. 7-8
- Codex: Chaos Daemons (6th Edition), pg. 8
- Codex: Chaos Daemons (8th Edition), pg. 8