Drow, of course, are a given. Players can look forward to squaring off against the race's nefarious priestesses and mutated dryders while they explore the city of Sschindylryn below the King's Forest of Cormyr. A number of deadly creatures await curious adventurers, including a new dragon for players to hunt. For the full details on what to expect from the Underdark, just check out the gallery below and then click past the cut for the full video.
[Source: Turbine press release]
[Update: Turbine likewise announced today that the expansion is in open beta, so head to the forums and jump in!]