Pandemonium

Pandemonium is an outer plane that represents the space between chaotic neutrality and evil.
Pandemonium is a harsh and rocky environment with four layers to it and most of it has not been explored by mortal, deity, or demon. While each layer is slightly different than the last, they each share a couple of similarities. Most obvious is its large and open areas with jagged mountains that are dotted with caves and caverns all throughout. The wind blows extremely harsh when out in the open with the sounds of everything in the plane all at once. Each layer would take a month of spelunking and exploration to reach the next.
Pendesmos
The first layer known as Pendesmos has several portals in the open spaces to Limbo which is where some would assume the harsh wind and assault of sound comes from. On the inside of the caves there are portals that lead to the Abyss. While Pandemonium is not very kind to any living being, this top layer is the most habitable of the four. Both in the caves and out in the open expanses there are ruins and temples of long lost and forgotten empires that have crumbled with age and exposure. The river Styx is also present in this upper layer also both in the caves and in the open. While it does flow through all of the seven evil planes of the wheel, nothing can follow the river from plane to plane as it often dissapears into the mountains and rocky crevices.
Cocytus
The second layer known as Cocytus, is similar but different than the first layer. The caves and tunnels are narrower, the caverns tighter and smaller. While the wind in the open is the same, there is no escape from it while in the caves. Sorrowful wales and shrieking cries would be a constant maddening experience without proper protection. Unlike the first layer where the caves are natural and random, in the second layer it is very clear that they were carved and directed by a creature that not even the gods know of.
Phlegethon
The third layer, Phlegethon, is known as the dark and damp. It is so deep here that the stone is venta black. Any light in this layer would only give the smallest amount of illumination, not even dark vision would be able to aid any being here.
Agathion
The fourth and final level Agathion, is the deepest and most inhabitable layer of Pandemonium. Here the ground is solid rock with no tunnels or caves. There is no wind as there is no air save for large bubbles or pockets of air that is in a constant and furious torrent of air. The current is so strong that it could fling even large creatures many feet up or away.
It is believed that Torog God of the Underdark has claimed this plane as his dominion. The only natural inhabitants of this plane are the spirits of the most cruel of under dark beings after they die. Shadow demons run amuck and attack any living thing they can sense. Quasits, low to mid level demons, and the occasional demon lord will venture into the caverns for their own purposes. Any other kind of being that can be found in any of the layers are due to exile as it is a common practice among the various planes that have the power to do so, there aren't many worse planes of existence to be banished to and therefor it is resereved the truly despicible entities.