The realm of Lolth, the Spider Queen, the Underdark is made up of a complex series of interconnected tunnels. True to its name, there is nearly no light in the Underdark; the Drow, who make up the majority of the sentient population, can all see in the dark. The Underdark is unnervingly warm and damp, and is home to many dangers, both flora and fauna.
From the Underdark, you can reach the Shadowfell and the Faedark.