The Abyss

A plane usually only whispered of in Academic circles. Whilst the other planes are wrapped tightly around the world, this plane is usually considered (when it is considered at all) to exist outside the world. That is; the other planes are Material-plane adjacent, but the Abyss exists outside it.
Some legends do describe it as a place of much fire and brimstone, although scholars attest that this is a corruption of legends of The Hells or even the Plane of Fire, which have been much more recently accessible, and home of the similarly evil Devils. More nuanced storytellers spin yarns of fantastical ever-changing landscapes, diverse in appearance; which seems far more in keeping with the Plane's inhabitants.
The Abyss is home of Demons - beings of pure Chaos, supposedly fighting endless wars against Devils, Angels, the universe itself, and each other? - Such things are unclear. Legends of those that have traveled here, tend to tell of them not returning, or returning quite changed. But little is really known of it and records of how to travel there have fallen out of common knowledge.

Type

Dimensional plane