The Primal Deities, also called the Dark Gods or the Nightmare Lords, are powerful entities who inhabit and control the psychic dimension that underlies all physical reality known as the Warp, created and sustained by the emotions and collective desires of every sentient being of the material universe. Although they are god-like beings, the Primal Deities are dependent upon the emotions of mortal creatures, especially the hordes of humanity, for their power and continued existence. As a result, the Dark Gods strive to convert all life to their 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 when the dimensional separation between realspace and the Warp broke down in its wake.
The Primal Gods and the Forces of the Warp that serve them are the greatest enemies of all life. Humanity and most other sentient forms of life in the Milky Way Galaxy, even if they are not fully aware yet of the threat, like the Solanae Imperium. The Dark Gods are able to devote a fraction of their psychic power in the Warp to create Avatar, whose appearance and character reflect their patron God's own nature. These Avatar may be reabsorbed into the God at their whim.
In the Warp, the psychic reflection of similar thoughts and emotions gather together like rivulets of water running down a cliff face. They form streams and eddies of anguish and desire, pools of hatred and torrents of pride. Since the dawn of Humanity, these tides and waves of psychic energy have flowed unceasingly through the mirror realm of the Warp, and such is their power that they forced creatures made of the very stuff of dreams and nightmares.
Eventually, these instinctual, formless entities gained a rudimentary consciousness of their own. The Primal Gods were born -- vast psychic presences composed of the fantasies and horrors of mortals. These are the Warp Powers, and each one is a reflection of the mortal passions that formed them. First amongst them is Rashna, the Lord of Battle, possessed of towering and immortal fury. Xhul'Horac, the bizarre and ever-changing Architect of Fate, weaves powerful sorceries to bind the future to his will, whilst great Zakuun, the Lord of Decay, labours endlessly to spread infection and pestilence. The last of their number is Socrethar the Eternal, the Dark Prince of Nightmares, indulgent of every horror and excess, no matter how immoral or perverse.
As the intelligent species of the Milky Way Galaxy prospered and grew, so too did their hopes and dreams, their rage and wars, their love and hatred. This burgeoning flood of raw emotion fed the Primal Gods and nurtured their power. Eventually, after the defeat of their servents by Humanity. The gods reached back, into and through the dreams of mortals, eternally working to influence the physical realm and its myriad sentient races to secretly dismantle Humanity and to rebuild the Soulless in the fringes of the Galaxy.
A Dark God can only grow in power through the actions and thoughts of mortals. Those who worship a Dark God, and behave in a way that feeds it, are rewarded with strange, extraordinary powers. As the Primal Gods battle in the Warp, so their mortal followers wage war in the material universe. The victors of the battles earn more power for their unworldy master, though the twisted plans of the Primal Gods are such that often victory is not necessary; merely the acts of sacrifice and battle themselves. When devotees of the Warp die, their do not fade in the Warp and disappear like the spirits of others to some unknown and unknowable fate. Instead, their immortal energy is swallowed into the greatness of their gods, their souls sustained forever, bound to the eternal power of the Dark Gods.