The Dead Lands of Athas
The Gray is a dreary, endless space, the place Athasians go when they die. This infinite expanse of ashen haze attracts the spirits of the dead once they give up the confines of their mortal raiment. In the Gray, the dead exist in a sort of limbo, floating aimlessly in the ether or assuming the forms they held in life. Eventually, the spirits of the dead are dissolved and absorbed into the Gray, much as their corpses are slowly obliterated by rot and decay on Athas.
Some spirits don’t suffer this fate. They are sustained by a force more powerful than the Gray—their will. Be it faith, trauma, or a lust for revenge, these spirits return to become the undead. Spirit undead (also called noncorporeal undead) can freely pass from the Gray to the material world of Athas, but they must return to the ashen plane at regular intervals to sustain their existence. Corporeal undead can’t enter the Gray, but they still draw sustenance from it.
Necromancers reach into the Gray for energy to cast spells. This source of power gives these wizards a compelling connection to the dead and undead, a connection that, in turn, influences the wizards. They have the look of death about them; it hangs like a dark aura or burial shroud around them, marking them as death wizards to those who can interpret such signs. Others simply feel ill at ease in their presence.
Though the Gray is dominated by negative energy and undead spirits, the Gray netherworld is also used for another reason. By piercing the dimension briefly at varying angles, one may use the realm for instantaneous transportation. Nearly all forms of teleportation make use of the Gray in this fashion, though similar methods have successfully been harnessed using one of the other realms such as the Black or one of the Elemental Planes.