The Wraith's Vengeance

Background
While exploring the Deep Roads underneath Vigil's Keep, you will eventually reach a room where a Shade will appear. Defeat it and the the Skeletons that it summons to begin this quest.

Walkthrough
Defeat the wraith in combat, then active the "strange apparatus" to release the creature. Continue down the path and battle the Ogre Commander and the other darkspawn there. Immediately afterwards, defeat the Possessed Ogre Commander (this is a very tough fight, so be prepared).

While this may appear to end the quest, that is not the case. Once you return to the keep, enter the basement once again and descend into the Howe family crypt (or tomb). Defeat the Avvar skeletons, then insert each key into the correct lock (you must have all four keys from the Deep Roads section). Proceed through the final door and battle the three Avvar Lords (another tough battle, so bring a healer) to finish the quest.