Eye of Terror
The Eye of Terror formed when the decadence of the Eldar civilization reached its peak, resulting in the birth of the Chaos God Slaanesh. This event cause a massive eruption of the Immaterium into the material universe, engulfing the core worlds of the old Eldar civilization. This massive "warp storm" has continued to exist for millenia without subsiding.
Following the defeat of the Chaos-corrupted Space Marine chapters at the end of the Horus Heresy, the Traitor Legions retreated into the Eye of Terror, which serves as a base of operations for them that Imperial forces can not safely attack.
Within the Eye of Terror, the Warp and physical reality are intertwined. The flow of time is inconsistent, the form of matter can be altered by thoughts, and daemons can roam freely.