Chronicles of Khaldun: Crux of Eternity
A dark reflection to the World (sometimes called the Prime Material Plane by scholars), the Shadowfell is a plane that forms the source of shadows and necrotic energy. It is a place of transition for dead mortals ready to be judged by the Raven Queen before moving to the realms beyond. Pieces of worlds that could have been, as well as entropic reflections of the world as it is, are said to reside here.
Some living souls are native to this plane (no doubt descended from ancient immigrants), but mostly the unresolved dead reside here.