Demiplane

Demiplanes were ill-defined planes of unusually small size that lay either within other planes or outside the known planes depending on which cosmological model one used. Demiplanes were typically, if not always, of finite size.
* In the Great Wheel cosmology, demiplanes were found in the Ethereal plane because they were formed from the swirling proto-matter that made up the Ethereal. The Demiplane of Shadow was the best known example.
* In the World Tree cosmology, there were a few demiplanes that were part of astral dominions such as the demiplane of Sildëyuir.
* In the World Axis cosmology, demiplanes could exist within other planes or outside of either the parallel planes or the fundamental planes. There were undoubtedly several of them, but most were unknown. Aside from those that lay within the dominions of the gods, the best known was Sigil, also called the City of Doors, which acted as a strange interplanar nexus with hidden doors to every known plane—and many that remained largely unknown.forgottenrealms.fandom