In Lioneye's Watch, talk to Tarkleigh to receive the Mercy Mission quest. He'll ask you to retrieve a medicine chest from the Submerged Passage. Head out of town and follow the coast to the Submerged Passage entrance. Be wary of the Drowned and other aquatic foes. The chest is located near the end of the passage. Return to Tarkleigh for your reward, which often includes a useful skill gem or item.
