Abyss

The Abyss embodies all that is perverse, gruesome, and chaotic. Its virtually endless layers spiral downward into ever more appalling forms.
Each layer of the Abyss boasts its own horrific environment. Although no two layers are alike, they are all harsh and inhospitable. Each layer also reflects the entropic nature of the Abyss. In fact, much of what one sees or touches on the plane seems to be in a decaying, crumbling, or corroded state.

Geography

There are infinite layers of the Abyss, and each offers a different perspective on the manifestation of evil.

Getting There

The abyss and the demonic horde within it keep trying to reach every other plane, invading them and corrupting them. Portals spawning all across the planes, dark rituals and sacrifices creating them with relative ease, and more ancient portals unable to be sealed such as the one within Redkeep's wall. The Abyss boasts more portals and gates across the planes than most other planes.

Traveling Around

There are so many ways for the Abyss to hinder travel that it’s difficult to document here. For the most part, hazards to travel equate to hazards to life. For example, water is rarely just water in the Abyss. It could be acidic, freezing cold, boiling hot, toxic, gelatinous, life-leeching, soul-sucking, screaming, or all of these things at once. The same goes for the air or the land.
Travelers need to be wary and learn as much about their abyssal destination as possible before traveling. Sometimes, however, this isn’t possible, especially if thrown into the Infinite Layers.

Creatures & Denizens

Powerful & Mighty

Demons are the most common inhabitants of the Abyss and their number is incalculable. They are ruled nominally at least by demon lords, and these monstrously evil foes offer the most tangible obstacle to completing goals in the Abyss. However, they are not the only ones with a vested interest in these infinite realms of chaos and evil.

Demonlords

Demons, occupying every layer of the Abyss, collectively form a great teeming horde of monsters from the darkest pits of blackest nightmare. Demons often take the form of twisted Material Plane creatures, though some have been transformed so completely they lack any basic commonality to known beasts.
And towering over the demonic horde are the demon lords. Each is a unique expression of evil made manifest in the Abyss. Some planar scholars argue that they are actually the physical manifestation of the Abyss itself, an explanation that offers justification behind their uniqueness.
Each demon lord holds god-like power on their layer, and it is thought that in their place of greatest strength they are effectively immortal. Outside of their home layers, however, they are more vulnerable, and some demon lords have been destroyed over the countless centuries. Others have been deposed, their influence waning with the loss of their layer.

Notable Demonlords

Ashlesh

Baphomet

Batara

Demonlord

Gazh

Juiblex

Salathiir

Tal Iss

Tchernobog

Tharizdun

Zyra

Denizens

Without a doubt, the most common creatures encountered in the Abyss are the innumerable demons. From quasits to vrocks to nalfeshnee and even worse, demons stand at the top of the Abyssal food chain. However, infinite layers spawn infinite possibilities, and other creatures have sprung up. Most of them are animalistic, never challenging the demons but instead picking off weak members and serving as food for other creatures.

Fiends

The most common inhabitant of the Abyss are the fiends, and they come in an astonishing variety of sizes and shapes that can only be produced by the utter malevolent chaos of the plane.

Demons

Without a doubt, demons are the most numerous and powerful fiends on the Abyss. The demon lords that rule over them are unique beings that were either created different from their birth or rose through the ranks to carve out territory for themselves. Demons embody the willfulness of chaos and the depravity of evil in everything they do, and the wide variety of demonic types that fill the infinite layers shows that there is a limitless supply of both in the multiverse.
Demons are a force of raw power and they live and die as the forms they start with; some demons are just created more powerfully than others, and that is the distilled essence of chaos. Nonetheless, lesser powerful demons can still serve useful purposes, and demon lords have special ways of rewarding others that perform tasks admirably. Of course, they may destroy such demons instead, perhaps to set an example or simply on a twisted whim.
Most demons are obsessed with simply spreading chaos, horror, and fear wherever they go, and most have little forethought beyond these immediate desires. They are a tidal wave of terror that threatens to spill out of the Abyss and engulf the multiverse, but an unlikely source has risen to stop them – the devils of the Nine Hells. The long-standing Blood War between the two fiends has been raging for centuries and centuries as the regulated armies of the devils face off against the wild hordes of the demons. This plays out largely on the first layer of the Abyss, the Plain of Infinite Portals, where the River Styx carries countless devilish troops and the crimson ground is soaked with blood of many fiends.

Humanoids

Few humanoids live in the Abyss by choice. Most are slaves of the demon lords, kept in prisons or cells as cattle to feed the endless demonic hordes, but a few communities actively serve the will of the greater demons and demon lords. These mad cults seek to please their masters, though few demon lords even bother noticing them, and any type of humanoid could have been brought up in a brain-washed cult of demon worshipping.

Tieflings

Demon-blooded tieflings rarely last long in the Abyss. The more powerful cambions see them as bastards and traitors with more “mortal taint” than demon blood, but some tieflings manage to prove their worth to the demon lords and greater powers. Often times, this worth is as spies, assassins, and elite warriors outside the Abyss.

Plants

The plant life of the Abyss is hostile, like everything else, but some of it is unique to the landscape of the infinite layers.

Features

Abyssal Corruption

A non-evil visitor that finishes a long rest in the Abyss must make a DC 10 Charisma saving throw. On a failure, the creature becomes corrupted. Refer to the Abyssal Corruption table to determine the effects of this corruption. After finishing a long rest, a corrupted creature can make a DC 15 Charisma saving throw. On a successful save, the corruption effect ends. A dispel evil and good spell or any magic that removes a curse also ends the effect.

Abyssal Corruption