The Abyss

Information from: sourceThis is the Birthplace of Demons. The number of layers in the Abyss is unknown and perhaps unknowable. Each layer has its own characteristics, which are as varied as the demons themselves. All types of terrain, atmosphere, weather, gravity, and material composition are represented in the myriad layers. 
Some examples are:
-Air-dominant—zero gravity, no matter except air.
-Ash—blowing dunes of ashes.
-Battleground—Blood War or other.
-City—warrens and slums to citadels and palaces, teeming with demons.
-Earth-dominant—solid rock tunneled by the inhabitants.
-Fire-dominant—everburning flames with salamanders, etc.
-Delusion—appears to be Normal but the flowers are poisonous or the trees attack.
-Desert—endless tracts of black, white, or red sand with no water.
-Glacier—infinite black, white, or orange ice with few rocky peaks breaking the surface.
-Hellscape—a burning mix of magma, rock, and belching brimstone.
-Maelstrom—swirling, bubbling chaos like Limbo.
-Mountainous—rocky crags, precipitous cliffs, soaring peaks.
-Negative-dominant—major or minor.
-Normal—similar to a Prime plane with various flora and fauna.
-Ocean—a trackless ocean, with a surface.
-Salt—crystalline badlands or desert.
-Sea—of acid, blood, garbage, insects, oil, or worms, etc.
-Swamp—noisome bog inhabited by predators.
-Volcanic—lava flows, earthquakes, volcanoes, and smoke.
-Water-dominant—nightmarish creatures in the murk.