Sol Udar, or the Realm Below, is the ancient, ancestral home of the Dwarves in Khyber. The modern Mror Holds clans are made up of groups that were exiled to the surface, and the way to Sol Udar was only opened back up during the Last War, where the dwarves discovered that their ancient kingdom had fallen.
Sol Udar is now in ruins, and those ruins are inhabited by aberrations; the forces of Dyrrn, the Corruptor.