Warning: I really couldn't find any official settings related to Chaos. My own additions are piled high.
The past.
The Warp.
Raging waves churned, lights swaying. This tiny patch of space was like a lone little boat amid towering waves, drifting helplessly and alone.
Even though two gamblers on the level of Chaos Gods sat upon it.
No, more astonishingly, amid such chaotic loss of control and fate, within a Warp that was abruptly reforming, this region of the Warp could still preserve its basic appearance—the only place that remained intact, while everything else was swept into the turbulent currents, rising and falling with the game upon the gambling table.
Pale white light illuminated the small square table.
With a clatter, The Emperor—or perhaps the Dark King—made the first move. That neither-human-nor-ghost, neither-gold-nor-black thing shoved the mountain of chips before him forward. The heaps of glittering, coin-like golden chips crashed onto the table, gleaming beneath the lights with a thousand watching eyes.
On the other side of the table, another greedy hand reached out, trying to touch those glittering little things. The hand was gaunt and skeletal, and ever-changing Lan Yu feathers flashed with countless colors beneath the light—
Hiss! Flames abruptly erupted across that greedy arm. The Dark King/The Emperor/Neos raised an disapproving brow, and Tzeentch immediately withdrew her hand. The Lord of Change lurked in the shadows beyond the light, her gaze sharp and bright.
I stake everything.
The Emperor/The Dark King said, black pupils almost completely swallowing the whites of his eyes.
Myself, the Primarchs, the Imperium, the species.
He murmured, the edges of the golden chips cutting through the air with sharp golden light.
Show me your sincerity.
Hiss.
The blue creature retracted her hand, then abruptly flung another huge pile of things onto the table.