From the Front Cover:
When Fort Iron was taken from the duergar who inhabited it, much of their subterranean quarters were never fully explored. An ancient oubliette has been discovered and its contents have disturbed the miners. Can the mine and the miners be saved from the dangers of the Oubliette of Fort Iron? A two-hour adventure for 1st-4th level characters.
From the Introduction:
Welcome to Oubliette of Fort Iron, a D&D Expeditions adventure, part of the official D&D Adventurers League organized play system and the Elemental Evil storyline season.
This adventure is designed for three to seven 1st-4th level characters, and is optimized for five 3rd-level characters. Characters outside this level range cannot participate in this adventure. A player with an ineligible character can create a new 1st-level character or use a pregenerated character.
The adventure is set in the Moonsea region of the Forgotten Realms, in the city of Mulmaster.