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.

Characteristics
According to earlier sources, demiplanes were planes still in the formation process, unfinished somehow More modern sources considered the demiplanes to be a catch-all classification for planes of generally measurable size and limited access, sometimes limited sufficiently that not even the ethereal plane or the astral plane were accessible, save from specific spots in other planes. They had great variety of possible conditions within. They were always extradimensional spaces.

Demiplanes could be naturally occurring, but most were created by deities or by wielders of magic. Demiplanes that touched upon the Wall of Color in the Ethereal Plane had portals of unique and consistent colorations leading to them.

Position in Cosmic Models

 * 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, though they lacked a set place in the Planes. 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.

Known Demiplanes

 * Cynosure: The meeting place of the gods, this plane contained only the Pavilion of Cynosure.
 * Demiplane of Imprisonment: Where Madryoch was imprisoned.
 * Grodd: The prison of Nalavarauthatoryl.
 * Hoard: A place hidden beyond Grodd.
 * Demiplane of Nightmares: Where the Night Parade became what it was.
 * Sildëyuir: The home of the Star elves.
 * The Night Realm: A splinter of Sildëyuir where a great evil was imprisoned.
 * Stardeep: Another splinter of Sildëyuir, this prison was in fact a demiplane.
 * Celestial Nadir: An ancient demiplane created by the Imaskari.
 * Manshoon's Demiplane: A demiplane used by Manshoon.
 * Silver Safehold: A demiplane created by Khelben Arunsun and the Sword Heralds.
 * City of the Dead: This cemetery in Waterdeep began using portals to demiplanes when the land ran out.
 * Black Abyss: A hidden realm once used by Martek as a trial.
 * Infernal Rapture: A demiplane ruled by a rakshasa named Mahadi.
 * World Serpent Inn: An interminable inn and tavern, later confirmed to exist in its own demiplane.
 * Tomb Tapper Tomb: An underground vault where tomb tappers placed all the magic they retrieved.
 * Myth Lharast: A city taken to a demiplane, which was only accessible when the full moon was directly overhead.
 * Cave of the Monkey: The hidden realm of Monkey.
 * Halaster's Tower: A hidden realm where Halaster Blackcloak kept a tower.
 * Demiplane of Fear: A location in which Dwalimar Omen discarded dangerous artifacts.
 * Genasi Refuge: A demiplane created by a well-meaning half-elf, Yrrick, as a homeland for genasi.
 * The Greenhouse: A demiplane accessible from Gheldaneth, where the church of Isis nurtured botanical research.
 * Wenimats: Where the gods of the Azuposi dwelt.
 * Land of One Hundred and Thirteen: A place created by Marek Rymüt.
 * Court of Stars: A demiplane ruled by eladrin.
 * Pocket Plane: A demiplane used by Gorion's Ward.

Demiplanes with a Contested Definition

 * Ravenloft was long considered to be a demiplane, but was also counted as being within the Shadowfell.
 * In turn, the Plane of Shadow was considered to be a demiplane before it was recategorized as a transitive Plane.
 * The demiplane of Time was characterized as such as a catch-all category for extraplanar locations.
 * The "demiplane" label was occassionally used for both the Quasi-Elemental Plane of Minerals and the Quasi-Elemental Plane of Lightning.
 * Sigil was considered a demiplane in some cosmologies.
 * Hidden Realm was either in the Outlands or in a demiplane, depending on the cosmology.
 * Matet was considered a demiplane as well as a seagoing vessel.

Nameless and Rumoured Demiplanes

 * Halaster Blackcloak kept another, smaller demiplane in the bowels of Undermountain.
 * Kaldrav used an old demiplane as a hideout and stash for the bandits of the Iron Wind.
 * Mezro was taken to a demiplane during the Spellplague.
 * Yûlash had ancient gates, some of which led to demiplanes.
 * The seelie and the unseelie may have dwelt in demiplane palaces.
 * Tempera may have been banished to a demiplane, per some rumors.
 * Sharn were, according to certain obscure lore, extrusions from a demiplane.
 * Sysquemalyn had created an extradimensional realm mimicking the Nine Hells with her sorcery.