Age of Heroes 5e
The Abyss, as its name implies, is an infinite, unending expanse of barrenness, despair, and evil. It is the birthing place of the Demons and home to the many Demon Lords. The appearance of the Abyss varies greatly from place to place; it is much like The Beastlands in that regard. The only constant of any terrain found in the Abyss is that it is harsh, hostile and seemingly lifeless. There are ranges of jagged, steep mountains; great expanses of dusty plains; boiling deserts; and ice fields exist in thicknesses of many miles.
The River Styx
The River Styx begins its evil flow in the Abyss at the Lake of Tears, where the Tree of Hate grows. The Lake of Tears is actually a receiving end for a conduit that originates in Gehenna. It is rarely traveled because the River Styx flows through its entirety. The River Styx flows through the Abyss in accordance to the terrain it's passing through, although it never freezes. Most of the activity that takes place here happens in proximity to its flow. Nearly all of the dark kingdoms of the Demon Lords are located within a few hundred miles of the Styx. Beyond these distances there can only be found exiles, passers-through, and those wishing to avoid the Demon Lords.
The River Styx flows into a sinkhole that leads to Acheron and from there, it flows deeper and deeper until it enters a portal leading to Tartarus.
The Lower Halls
The Abyss also boasts an underground realm that is as unending as the surface realm. It is called Acheron, The Lower Halls. There are few Demon Lords that make their homes here (Baphomet is the best known), but even they dwell near the surface; there are things unknown in the deeper areas of Acheron that even the Demon Lords avoid. The deeper halls of Acheron wind and twist in a chaotic fashion. One can never be sure which direction the surface lies as gravity is warped by the forces of chaos.
The Abyss is mainly populated by the demons, whose primary past time is battling each other and occasionally plotting to conquer the Earth and its denizens. Each Demon Lord claims a sect of the Abyss, marshaling their forces against each other.
There are few other inhabitants in the Abyss and they are relatively unknown. What is known for certain is that any creature found living here is exceedingly dangerous.
Plants, also, can be found in the Abyss, usually near a Demon Lords stronghold; plants in the Abyss are always partly sentient, being able to move their limbs and most are carnivorous.
Lastly, there can be found many zombies and ghouls of varying races in the Abyss, though they all are controlled by Orcus, for he is their Prince. While the other Demon Lords hold standing armies of demons, Orcus primarily commands the undead. The undead in the Abyss differ from the undead in other Planes, for here they have been turned to the will of Orcus.