From the Back of the Book:
No realm in the Dungeons & Dragons 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.
This Dungeons and Dragons game supplement contains everything Dungeon Masters need to run adventures or campaigns set in the sprawling underworld of their campaigns, including new monsters and hazards, ready-to-play encounters, monster lairs, and detailed information on various dark-dwelling "movers and shakers."