The Underdark is an expansive area beneath the Black Mountains. It's in a state of constant change, as quakes can build new caves that stretch out for miles. The Underdark is an extremely hostile environment that's home to many creatures, including Kobolds and Drow. Shops and safe places to rest are few and far between.
There are a number of interesting areas within the Underdark. A small Kobold compound can be found early in the journey. Rugnar's Realm is in the Underdark Tunnels. He has information on the surrounding areas, runs a small store, and has a nearby camp site for weary travelers. Rual'Thor's Home can be found beyond the battlegrounds. There's also Maeralssin, a large Drow city that's home to many powerful houses, such as House Do'Vrinn.