Dungeons & Dragons: Out of the Abyss Adventure - Rules of Play says:

The Underdark is a subterranean wonderland, a vast and twisted labyrinth where fear reigns. It is the home of horrific monsters that have never seen the light of day. 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!