Used to contain the Banderling Gertarh who sold Gertarh's Dagger, his throwing daggers and also armor until he was killed by Oswald during the Thorns of the Hopeslayer event. You can still find a key to access his den in the main dungeon but strangely it only unlocks doors beyond the tattered sign, you will need lockpick for the main door.
The bottom most part of the den, known simply as the pit, was once a popular levelling spot for low levels and friends to hunt at.
The Alloy Cogwheel that is used in the Undead Mechanic quest can be found in the pit at the very bottom of the dungeon (See Map). Numerous loot spawns, chests and even a corpse containing loot can be found in the den.
During the Reign of Terror event, the dungeon was updated with new creatures, the Alloy Cogwheel quest item and the level 8+ restriction was removed.
Alloy Cogwheel - found in the north room of the main pit
Chest (Tier 4) - multiple chests found in the pit area
Corpse of Gertarh - found in Gertarh's den, contains the mysterious note
Key (Thieves' Den) - found on the floor in the south west lever room. Unlocks some of the doors to access Gertarh's den and also one of the cells in the den itself.