The Abyss

(https://forgottenrealms.fandom.com/wiki/Abyss)
The Infinite Layers of the Abyss was the birthplace of the demons, a chaotic evil universe unto itself with uncountable layers of infinite variety connected haphazardly. The Abyss's place in the cosmology of the Forgotten Realms shifted over time. Still, the nature of the plane remained relatively constant: a violent, malevolent place where the strong survived on the backs of the weak, the weak conspired to overthrow the strong, alliances only lasted as long as they were convenient, and the landscape itself tortured the minds and bodies of all who dared to pass.

Layers:
The number of layers in the Abyss was unknown and perhaps unknowable, with at least 679 having been catalogued. Still, this article attempts to list them regardless of which cosmology model was in favour at the time of discovery. Each layer had its own characteristics, which were as varied as the demons themselves. All types of terrain, atmosphere, weather, gravity, and material composition were represented in the myriad layers. Some examples were:

Plain of Infinite Portals- Pazuzu
The Gaping Maw- Demogorgon
Thanatos- Orcus
The Demonweb- Lolth
The Endless Maze- Baphomet
The Triple Realm- Graz’zt
Death Dells- Yeenoghu
Hollow Heart- Fraz-Urb'luu
Shedaklah/Molor- Juiblex
Shedaklah- Zuggtmoy
Lovers Lake- Elrith

Despite its name and notoriety for being never-ending, it was not known whether the layers of the Abyss were truly infinite in number. Part of the reason for this was that some places might have been so inhospitable that none, not even mortal, nor god, nor demon lords themselves, could survive (the deeper layers reportedly having been even worse in the past), and hence were not known about.

While the hundreds of layers of the Abyss might seem to eclipse all other Outer planes put together utterly, this figure was misleading. Besides the issue of uninhabitability and the layers and divine realms, the personal territories of powers were often one and the same in the Abyss, whereas other planes frequently had several realms on a single layer. Exacerbating the problem, and another reason why the Abyssal layers could not be counted appropriately, was that the number was always in a state of flux.

In the very depths of the Abyss, the heart of the plane was constantly cleaving through the primal foundations of the cosmos, as it had since before the dawn of mortals, spawning new layers of existence. Arguably worse, if a demonic invasion was allowed to fester, entire worlds could fall into the Abyss, becoming the basis of new layers and realms. Conversely, the Abyss was also capable of losing layers. The individual layers of the Abyss had primal urges and a constant need for nourishment, shrinking and weakening until nothing was left if not fed. Alternatively, they might be absorbed by other layers, only able to escape this state by acquiring sufficient sustenance. Occasionally, the battling of demons over control of a layer could become so cataclysmic that the entire realm was outright obliterated.

Layers could also be intentionally removed, such as when Lolth expended much of her divine energy separating the Demonweb Pits, or when a pantheon of offended deities ripped out and condensed a demon lord's entire layer (with him still in it) and cast it into the Astral plane. Despite this ending in the Abyss, much like a hungry, rudely awakened beast, almost instantly striking down the entire pantheon in that case, the attack itself was successful, leaving nothing but a void and their own decomposing divine corpses.

It was also a possibility that destroyed layers were capable of being reborn elsewhere in the Abyss, although this was not a certainty. This ignored the fact that layers were shaped by the conscious manipulations and subconscious wills of their resident demon lord, and so even the nature of a single layer could become obsolete by the time such information was established.