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.
In the World Axis cosmology, conceptual aspects of the Plane of Shadow, the Negative Energy Plane and the Ethereal Plane would be amalgamated to create the Shadowfell.