The Underdark
An entire plane of existence that is the shadow of the Material Plane, twisted. There is no sun, there is no safety. This is kind of Dune, but Underground. all of the 21.15 Drow Queendoms are in the Underdark.
In this campaign the Underdark is not simply a network of caves under Faerûn, it is a separate plane. Its relationship to Shadowfell is unclear.