Undermountain: Halaster's Lost Apprentice was an adventure module for the D&D Encounters program in Spring 2010, for the 4th edition of Dungeons & Dragons.

A lost passageway on the upper level of the most infamous dungeon in Faerûn has recently been uncovered. When a patron hires a band of adventurers to brave the dangers of Undermountain, a search is on to uncover secrets of an apprentice of the dungeon’s lost creator – Halaster.

Undermountain: Halaster’s Lost Apprentice is designed for five characters of 1st level and is the spring 2010 season of the D&D Encounters official play program.