The Scourge of Icewind: Part 3

From Introduction:

At the end of Part 2, the PCs had traveled to Glacierdale, the long-abandoned town destroyed in the Spellplague, and discovered an opening to the Underdark. Exploring the deep, they discovered a large nest of drow and cyclops, presumably under the control of mind flayers. On their way out, they were attacked by more mind flayers. Searching the bodies, the PCs discovered a note written in Draconic commanding the mind flayers to destroy the PCs written by some creature known as Krellix. Returning to Horizon Reach, General Drull is told of the events, and he tells the PCs to prepare for the eventual coming of the mysterious Krellix.