The Shadowfell is a dark reflection of the Material Plane, a plane of death and darkness. In the taxonomy of Eberron’s planes, it is called Dolurrh, and more coloquially, the Keeper’s realm.

Dolurrh, the Realm of the Dead, is a hopeless gray waste where mortal souls go after death. It is not a reward, nor a punishment, it simply is. Dolurrh has heavy gravity, it is timeless and people who visit Dolurrh are slowly overcome with apathy and eventually fade, turning into a shade. When Dolurrh is coterminous to the Material Plane slipping occurs between the planes. Ghosts are more common and spells to resurrect the dead often result in bringing back more spirits than originally intended. When the plane is remote, resurrection magic does not work at all and the only way to bring a soul back from the dead is to physically travel to Dolurrh and retrieve the soul, though the native inhabitants of the plane don’t take too keenly to having souls being taken. The shadar kai are said to have originated here.