The Abyss

"The set of structures commonly called the Abyss is an indomitable abomination to our world" - Vequaniel, Supreme Lexiconist of the Heavens

Structure

The Abyss is a constantly evolving, chaotically shifting and hazardous plane of existence, its environments ranging from freezing glaciers to poisonous bogs and lands filled with fire and brimstone. It is often described as having hundreds of layers, each very different and with its own, mostly unique inhabitants who mostly consist of various types of demons. Within this layer model the topmost layer is labeled as layer number one, the Plane of Infinite Portals, which is demon lord Pazuzu's realm. From there downward, though in reality a lot of layers are adjacent to each other, span several hundred vastly distinct territories. Like previously stated, due to the constant changing nature of the Abyss, these layers also sometimes quite literally move within the structure. One major example of this is The Gaping Maw, which as a realm is in its entirety the enormous demon lord Demogorgon's water-filled abyssal abdomen as he hungeringly glides through the Abyss on his neverending journey.
Each layer has one or several demon lords who act as the ruling body with very often absolute authority over their domain and the residents within. The burden of this binds the demon lord's soul to the essence of the layer itself, and vice versa, rendering them largely immortal. If however, a demon lord is to be permanently slain, the domain itself 'appoints' a new master by applying the highest level of corruption to their soul, often changing them in numerous ways both physically and mentally to better fit their new position. Only a few times in recorded history has a new master of a layer been powerful enough to resist the change and instead turn it on the realm itself, bending it to their will.

History and creation

"Those involved in this universe of infinite evil deserve no mercy" - page 163 of the holy doctrine of Astair

Before the Abyss, the pit of raw chaos that lay in its space was called the Elemental Chaos. This endless pit would spawn monstrosities of an ancient demon race, the obyriths. The deities of the heavens spent eons fighting these creatures until with the decree of Pact Primeval delegated those duties and responsibilities to the devils of Asmodeus, while themselves focusing more on creation. The devils were an effective unit and albeit the infinite nature of the Elemental Chaos, were winning, until a corrupted deity by the name Tharizdun defected the heavens.
Tharizdun joined forces with the mightiest of the obyriths and used his powers of creation to mold a new home for the demonkind. This new home, the Abyss, was a more structured environment where chaotic behavior and the survival of the fittest was still encouraged under strong leadership and authority so that they weren't as easy pickings for the relatively disciplined devils. To accomplish this, he forcibly forged together aspects and whole regions of the elemental planes with the raw chaos of the pit, an act the primordials still hate him for to this day.
Tharizdun was largely successful. The demons soon gained a sudden upper hand as the unsuspecting devils had to adapt and change, sparking a long-lasting war called the Blood War. The corrupt god himself didn't get to be involved in the warfare, as he was promptly imprisoned for his crimes in an inescapable mirror-plane of the Abyss, the Voidharrow.

Inhabitants

"To us devils, it is an unforgivable insult to be called a demon." - Sciopus, Harbinger of Words

A permanent inhabitant of the Abyss is by definition called a demon. There are many different kinds, some with appearances more traditionally associated with demons, and others more discreet and hidden. An unfortunate visitor is of course likely to run into creatures with large limbs and appendages, spiked tails, sharp teeth and claws, but not everywhere are they a majority. In the realms of shadow such as the Midnight Isles, the most common residents are various casts of succubi and shadow demons, the latter of which possessing a largely incorporeal physical forms. Those demon lords most cruel also forcibly experiment with new breeds constantly, for both their own amusement and to gain advantage over the other lords.

Type
A plane of existence, chaotic evil
Population
Seemingly infinite
Ruler
Each layer has it's own ruler(s)