The Abyss (or Infinite Layers of The Abyss) is an Outer Plane and is home to all of demon kind. The Abyss isn't a true plane in itself – it is comprised of countless smaller planes, each of which is a realm overseen by a demon lord and their followers.
The Abyss is home to unimaginable horrors, and mortal creatures who spend any amount of time in the Abyss find their minds and bodies corrupted by the chaotic energies that pulse through its layers.
Travel
Travel to The Abyss is achieved through various spells, or rarely via The Astral Plane. Demons are unable to travel from the Abyss into the material planes without intervention from a mortal creature native to that plane, however demonic influences can push misguided mortals to do so through dark rituals.