Glenn's Eberron Campaign
Khyber is said to be the body of one of the three Progenitor Dragons that created the world of Eberron. It is a massive subterranean realm that is filled with Fiends, Aberations, Daelkyr, and sleeping Rajahs that were bound by the Dragons and the Gatekeepers.
Basically, the Underdark.
Khyber was one of the Progenitor Dragons.