Dishonored: The Knife of Dunwall DLC -
Climb up the roof and down on the other side. You'll find out that Lurk has been captured by some overseers. You can either shoot the Overseer with the device or drop a chokedust on them. If you chose to shoot the overseer, then equip a sleep dart and shoot his leg or body, don't aim for the head cause the mask will block your shot.