Abyss

The Abyss is cosmologically the lowest plane, except for the Negative Energy Plane. Its nature has changed greatly over time, namely due to the events surrounding the Concordance.

Pre-Concordance
The Abyss was once a vast and empty plane, infinite in its dimensions. It was akin to a void or bottomless, colorless pit. It seemed the perfect place to seal away the mad and rebellious god Anarcathus, and so he was placed there as part of the Concordance.

Post-Concordance
After the Concordance, Anarcathus was able to breach the barrier between the Abyss and Negative Energy Plane, feeding on the flow of energy in order to spawn an infinite hoard of chaotic entities known as demons. These demons are at first formless spirits that battle each other for supremacy within the Abyss, with only the strongest rising out of the pits and into other planes. Unlike Anarcathus himself, demons can freely travel to other planes.

Most demons rise into the Verge and continue to battle with one another. The most powerful of demons then travel to other planes, usually to Armageddon to take part in the primary battlefield of the Blood War. Some demons travel to the Elemental Planes, or the Material Planes if summoned.

Still other demons are gathered up by Pathos to act as his servants. Generally they are converted into more neutral forms to better serve their new master.