The Shadow Labyrinth Key drops from Talon King Ikiss in Sethekk Halls. It is available to everyone in the party who does not already have it. Note that there have been reports of a bug preventing players from opening the Coffer when they defeat Talon King Ikiss too speedily. High level characters should attempt to prolong the fight to last at least one minute in order to ensure access to the Key. Patch 2.3.0: "The Shadow Labyrinth Key can now be looted from the Talon King's Coffer next to Talon King Ikiss in the Sethekk Halls"