Undermountain: Stardock

Undermountain: Stardock, written by Steven E. Schend, was released by TSR, Inc. in 1996. It is the last of three Dungeon Crawl adventures to be set in the largest, most dangerous dungeon in the Forgotten Realms: Undermountain. The adventure is meant for 4-6 characters, levels 10-14.

"Had I not seen the chaos that ensued, I would never have believed the tale Shadowdale's sage wove for me that day... Who would have believed that anything had the power to rip Halaster Blackcloak bodily from his Undermountain? Who again could predict the maelstrom of dangers that a dungeon gone mad would unleash upon an unsuspecting Realms? It is thanks to the brave and noble souls who dared reach into the villains' clutches to save the life of Halaster the Mad Mage that we all rise to greet another sun."
 * – Gamalon Idogyr, starfaring mage formerly of Llorbauth

This DUNGEON CRAWL adventure returns once more to the hallowed halls of Undermountain, but it's a different day. At dawn of Highharvestide 1369, every practitioner of magic on the Sword Coast knew there was a problem. By midmorning, the Lords of Waterdeep confirmed the ill omens: Some brave souls worthy of the title "Hero" must enter Undermountain on a mission to save Halaster Blackcloak! Could these heroes be your group of player characters?

This is a stand-alone module and does not require extra support materials. It can be placed into a campaign on any AD&D world, but best fits and impacts a FORGOTTEN REALMS campaign set in Undermountain or Waterdeep."

This source book is now available as a free PDF from WotC's website.