Abyss

The Abyss is a vast wound in the cosmic order, a bottomless pit teeming with creatures that exist only to rend, tear, and destroy. The demon princes, individual demons of great power and determination, bend and shape the Abyss and its inhabitants to meet their every whim. These mighty beings imagine themselves at the center of the cosmos. Each demon prince believes that the universe will one day be theirs to command, its laws and structure twisted and warped to match the demons' ideal of perfection.
The physical nature of the Abyss is something that few mortal minds can understand. Those who discuss such matters use the term "layer" to define a certain part of this infinite expanse. That nomenclature, to the extent that it implies a particular configuration, is misleading. The Abyss is a chaotic tangle of miniature worlds, each one shaped by the demon lord that claims primacy over it. Within the layers it controls, a demon lord manipulates conditions to match its view of how that world best serves the lord's desires. Of course, keeping control of a layer involves fighting off other demons that are looking to expand their domains. From time to time, a layer changes hands or is seemingly obliterated in a battle (perhaps to be reborn in another location). For this reason, imposing a sense of order on the relationship between the Abyss's layers is a fool's errand. All that can be determined at any given time is which lord holds sway over which parts of the Abyss, and which areas are being contested by two or more lords. And as soon as such a fact becomes known, it might already be obsolete.