The Abyss

The Abyss is the home plane of demons and yugoloths. The plane is frequently associated with an infinite void and fire.

Layers of the Abyss
The different parts ruled by the various Demon Lords are called layers

Apollyon's Realm
Possibly the mightiest of the Demon Lords, Apollyon reigns over her layer of the Abyss from the great city of Eschaton.