In order to obtain this quest you need to do three things (in no particular order):
- Get the letter off the dead adventurer's corpse located down the first hallway on the left after entering the Ruined Temple accessible during or after The Urn of Sacred Ashes quest.
- Go to Tapster's Tavern in the Commons of Orzammar and talk to the nervous adventurer in the north-west corner. After some dialogue, he will give you a set of notes.
- Go to the Lower Ruins in the Brecilian Forest and acquire the journal from the bone pile of another dead adventurer. It is located on the left side of the large "fire trap" room in the Lower Ruins.
Once all three are completed, travel to the Dirty Back Alley in southeastern Denerim. Interact with the door to a Quaint Hovel which is the home of Vilhm Madon and ask about Gaxkang to initiate dialogue and a difficult battle. See Gaxkang the Unbound (strategy) for tips on how to defeat it.
Upon defeat, Gaxkang will drop the Keening Blade and several other items. There is a slight chance he will drop the Fade Wall shield and up to 5 0 0 on his body. In addition, you can find around 10 0 0 and Elementalist's Grasp in the chest in the back room.
After defeating Gaxkang, the Dirty Back Alley will disappear from the Denerim Map. The area will then be inaccessible. It is therefore advised to complete the quests Back Alley Justice, Blood of Warning, and Notices of Death prior to Unbound.