The Abyss

"Some scholars number and name the parts of the Abyss, declaring that this Demon Lord or that rules a portion of the place.  When your house is on fire, does it matter what the flames look like in each room?  Should you bother to give them names?  What's important is escaping and putting out any fire that clings to you." - Mordenkainen

From Manuel of the Planes (3e):

It is an infinity of clutching horror.

It is home to demons.

It is where morality crumbles and ethics perish.

The Abyss is all that is ugly, all that is evil, and all that is chaotic reflected in infinite variety through layers beyond count. Its virtually endless layers spiral downward into ever more atrocious forms. Conventional wisdom places the number of layers of the Abyss at 666, though there may be far more. The whole point of the Abyss, after all, is that it's far more terrible than conventional wisdom could ever encompass.

Each layer of the Abyss has its own unique, horrific environment. No theme unifies the multifarious layers other than their harsh, inhospitable nature. Lakes of caustic acid, clouds of noxious fumes, caverns of razor-sharp spikes, and landscapes of magma are all possibilities. So are less immediately deadly terrains such as parched salt deserts, subtly poisonous winds, and plains of biting insects.

The Abyss is home to demons, creatures devoted to death and destruction. A demon in the Abyss looks upon visitors as food or a source of amusement. Some see powerful visitors as potential recruits (willing or not) in the never-ending war that pits demons against devils: the Blood War.

Demon lords and deities inhabit the Abyss, including Demogorgon, Graz'zt, Pazuzu, Blibdoolpoolp (deity of kuo-toa), Diirinka (deity of the derro), the Great Mother (deity of beholders), Gruumsh (deity of orcs), Hruggek (deity of bugbears), and many others, including the well-known deity Lolth (drow deity and queen of the Demonweb Pits). Other demon princes include Yeenoghu, Alzrius, Baphomet, Eldanoth, Fraz-Urb'luu, Juiblex, Kostchtchie, Lissa'aera, Lupercio, Lynkhab, Pale Night, Verin, and Vucarik.

As noted before, the Abyss has layers beyond count, though the top layer is well-known: the Plain of Infinite Portals.

The Abyss has more layers than anyone knows, mortal or deity. A few of the most well known are described below.

Vision in the Abyss is normal except on layers where environmental conditions inhibit it. Unless noted otherwise, hellish suns, phantom glows, or similarly unpleasant radiances illuminate all layers of the Abyss.

Unless a particular layer somehow inhibits sound, hearing is also normal in the Abyss.