Delve into the subterranean Underdark in this D&D adventure rulebook for character levels 1-15! Enter a vast and twisted labyrinth where fear reigns and horrific monsters reside. It is here that the dark elf Gromph Baenre, Archmage of Menzoberranzan, casts a foul spell meant to ignite a magical energy that suffuses the Underdark and tears open portals to the demonic Abyss. What steps through surprises even him, and from that moment on, the insanity that pervades the Underdark escalates and threatens to shake the Forgotten Realms to its foundations. Stop the madness before it consumes you!