Plane of Shadow
The Plane of Shadow is a late addition to the Great Wheel cosmology of Dungeons & Dragons, having originally existed in Advanced Dungeons & Dragons 2nd edition as the Demiplane of Shadow. Essentially, it is akin to the Ethereal Plane in that it lies coterminous with the Prime Material, but it is defined as looking like a lightless, colorless mirror of the Prime Material.
The Plane of Shadow is a major part of Urban Arcana. It is the source of all the supernatural creatures that are appearing in the "real" world without memories of their homeworld and why planar transportation magic doesn't work here. This is referenced in the third edition The Manual of the Planes.
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes|
|Elemental Planes||Energy Planes||Demiplane of Dread||Plane of Shadow||Plane of Mirrors|
|World Serpent Inn||Tu'narath||Sigil||Demiplanes||Ordial Plane?|
|Great Beyond of Pathfinder|
|Astral Plane-Ethereal Plane|
|First World-Inner Planes-Shadow Plane|