Realm of the Underdark held by the Aboleth
The lowest known realm (though there may be others below), completely flooded and held by the Aboleth who do not allow any other species in. The true extents of Melaskos are purely conjectural as no non-Aboleth has ever explored there in detail.
The Kingmaker was located in an air-pocket on Melaskos, guarded by four fire demons and the Death Knight Asildur.