The Abyss

The Abyss is an outer plane that is the embodiment of chaotic evil.
The size of the Abyss is an argument up for debate and faith. The Abyss is home to an uncountable number of independent demi planes that are equally unique and different with their own terrains, entities, and oddities. It helps to think of each plane as a layer on top of the next, meaning you could not enter the 3rd from the 1st, you would have to travel from the 1st, through the 2nd, and you would enter the 3rd. Occasionally, there will be a layer with multiple entrances to multiple other planes. It helps to imagine these creating new stacks of layers such as 7A or 9D for example. It is considered the home of demon kind. If a being, usually a demon, is able to learn and control at least one of the layers would mean that they are powerful enough to be considered a Demon Lord, many of which are believed to have once been some of the lesser gods during the 1st era.
During the first era and before the Calamity, a group was formed in the hopes of exploring this plane. The Order of Consternation was organized from members of all the other deities. It was this order that began to number and chronical the layers as they went further down. At first the expedition would consist of one scribe, several warriors and clerics, and a varying number of Paladins. As their travels went deeper however, the expeditions required a force of legends to continue. At its prime, the order's expedition would consist of a small town's worth of people. Miners of Torog as some layers needed to be excavated, shelters built, and geometric activity analyzed. While they rarely ventured into the plane itself, keepers of Zanthora would spend years financing and preparing the rest. Members that followed Satirii were especially needed for the layers that were desolated and barren. Most of the Paladins would have come from Mauros and his heavenly host. Those that followed Sehanine and/or Kord provided much of the provisions for the travels. A school of scholars and scribes from Erathis to record all of their findings. Clerics of both Amara and the Raven Queen were some of the most respected and well treated members. Scores of warriors from both Gruumashkt and Azerus was the bulk of the order.
There are a few chronicles that have survived to the modern era detailing a few levels. 1. The first layer is the most well-known and called The Land of Infinite Portals. It is the only plane that is "neighboring" the material plane. This layer consists of a dusty wasteland, the river Styx, iron fortresses, pits leading to deeper layers that are rarely used by visitors from the material plane, and portals that lead seemingly anywhere in the material plane. It is believed that many of the horrible monstrosities that enter the material plane do so from here. 2. The Hive. Ruled by a Demon Lord, the Chattering Prince, very little recordings of this plane have survived other than a famous etching that has the word, "BUGS" that was written furiously and harshly. 3. The Forgotten Land. Ruled by the Ungrasped. A deal eventually had to be made with the order and demon controlling it as this demon was fond of stealing memories. 4. The ruling of this layer was in contest. Notably there were more portals to various other layers but you had to scale down a severely large pit that seemed to be neverending 5. Wormblood. another layer under contest for control. Notes on this layer mainly are about the quickest way to get through it as it would infect beings with horrible incurable disease 6. The Realm of Eyes. This layer was in a definite state of control by the Watcher. The mother of all Beholders. This layer is a series of tunnels that have large living eyes that were connected to the mother. 7. Skin-Shedder. Home of a demon Lord and his army of lizard like beings. 8. Phantom. When the expedition stayed too long in this plane it began to corrupt the souls leading them to a more chaotic or evil alignment 9. BurnWater. A layer of an acidic ocean with seldom land 10. The Grave. What seemed to be the remnants of a battlefield of colossal sized beings. The only remnants of them were mountainous bones.
At a time, it was once believed that the last layer was 666. In the Modern era however, it is commonly believed that some demon lord has sealed the portal further down or hidden it away. Some historians like to theorize whether it was to seal off a greater evil or hide away some treasure or power.