Underdark (4e)
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.'