Put simply, the Underdark is the labyrinth of caverns, tunnels, and underground lakes that stretch under the whole of Faerun. It’s home to countless creatures, magical and otherwise, and offers few protections for regular surface dwellers while also promising great power for those who can navigate its depths. Check out the full article for more details on this home of Drow and Illithid alike, the great underground wilderness.
Neverwinter expansion is sending players to the Underdark. All the way to the Underdark! Which is great if you know what the Underdark is and significantly less great if you’re mostly of the mind that it sounds vaguely like a metal band name. Not to worry, though; the official site has you covered with an explanation of just what the Underdark actually is, something that players might not have realized even with earlier adventures exploring bits of it.
Source: Official Site