The Last Stand
From Diablo Wiki
The objective of The Last Stand quest in the Old Smugglers' Warehouse is to help the guards protecting survivors by killing the reapers attacking the warehouse. These reapers are composed of a Punisher and many Revenant Soldiers. Once you are done with the slaying, talk to Captain Nowell. You then get your quest reward (XP and gold) and you'll be able to enter a smaller room containing a Resplendent Chest.
The Last Stand is one of the quests that must be completed in order to get the Sightseeing and The Tourists Achievements.