This quest is unlocked upon reading a note from a table early in The Descent quest line. The note forms part of a crazed dwarf’s journal. As note 1 of 4, the other three can be found throughout the darkspawn warrens and the ruins of Heidrun Thaig.
The second note is in a room just before you proceed through the door with the missing gears in the darkspawn warrens, in the same room where Builder’s Towers is acquired.
The third note is inside the eighth Sacrificial Gates of Segrummar, which can be found in the Ruins of Heidrun Thaig near the central bridge.
The fourth note is in a small cave within the Ruins of Heidrun Thaig. The cave entrance can be found just before the chamber where you fight an Emissary Alpha and Ogres.