Blood and Jazz
A demiplane is a minor plane of existence, usually artificial and of finite volume. Very small or unstable demiplanes may be crafted by mortal spellcasters and concealed within magic items (such as bags of holding, portable holes, or rods of security), but demiplanes that can support life indefinitely are usually the sole province of deities and of the rare casters who have expanded their magical knowledge to epic levels.
Ërdin: A demiplane created by the deity Boccob, it is notable for its particularly great size (a sphere of radius roughly 1.2 million miles), its Material Plane-like traits (located at the center of the Great Wheel, possessing an associated Shadow and Ethereal plane, etc.), and the potent abjuration that prevents non-natives from traveling to it or summoning natives from it under their own power (Boccob’s Veil).