"Ah, the Underdark. How I love this place. Please keep your arms and hands to your sides at all times. Except when we encounter a drow patrol that is firmly set on making a Cael-coat - Then I expect you to start hitting them. - cael-am in Ser'chel.
Generally the Underdark is a large series of caves and tunnels, deep below the surface. Light is virtually non existent this far below, as are winds. The climate is constant, with no weather patterns or sense of day or season.
The Underdark is full of dangers, as only the most hardy survive for an extended period of time. Among the most known, are the Drow, the Duergar and the Illithid (Or mindflayers).
Below Thain lies the Underdark as well. The drow claim the north with their city Mora'chel. Their territories stretch from there, to the south where their borders collide with the duergar and their city Chudrak-dûm.
Most lore on the Underdark is unwritten and passes from mouth to mouth.
Raids or trips to these areas are quite frequent, though with little success in weakening the city-states. The most successful to date, was the recovery of the King of Hammersong.
The drow and duergar both only have a single city, though some evidence of a second drow city and ruling house, has been recovered in the numerous raids on the Ser'chel barracks.