Underdark (4th edition sourcebook)

"''Descend into the depths of the world.

No realm in the D&D world stirs the imagination like the Underdark. This vast subterranean domain holds thousands of adventure possibilities and myriad threats, including drow, mind flayers, dragons, and worse! Entire campaigns can unfold in its depths, and its rewards are boundless.

Underdark contains everything a Dungeon Master needs to run adventures or campaigns set in the vast underworld of his or her D&D campaign, including new monsters and hazards, ready-to-play encounters, monster lairs, and detailed information on various dark-dwelling "movers and shakers.""