In The Abyss virtually everything-and everyone-that can be fought over, is fought over. Layers are often sites of centuries-long battles between demon atrocities, much less the greater wars of demon princes such as Demogorgon, Graz'zt, and Orcus. Multiple demon princes even fight with each other over which one of them should get the exclusive freedom to destroy the rest of Creation.
The newest release of the unofficial Horrors features four previously revealed layers and one new layer of The Abyss, each of which has seen strife and conflict from a wide array of powers. They are:
Pazunia: The first layer of The Abyss, Pazunia has a far older history than most have ever suspected. The caverns of Broken Reach offer a semblance of hospitality, while Styros offers little else but death and strife.
The Grand Abyss: One of the other great byways of The Abyss, The Grand Abyss is an ancient remnant of far older and more bizarre powers.
The Wells of Darkness: The Abyssal analogue of a prison, The Wells of Darkness began as an experiment by Demogorgon and have played a role in the histories of practically every major Abyssal lord, from Graz'zt and Vuron, to the War of Ripe Flesh, to Zuggtmoy and the qlippothic war against the rebellious klurichir that followed Laz'bralthull.
Durao: A stagnant and highly fractured layer that has mostly resisted the attempts by any power to seize control of it, the Styx-flooded layer named Durao is the site of eternal conflicts between the Abyssal lords and those who follow them.
Cogerron: A bizarre and extremely hazardous labyrinth so ancient that even the qlippothim cannot recall who once ruled it, The Pathless Nightmare holds rewards through artifacts and knowledge and myriad hazards from the unique demons that the layer spawns to the dangers of its innate properties.
(edited to add Durao)