You cannot return to finish this quest after beating the main story
You get the quest when you go to
In order to finish this quest you need to destroy boxes of orichalcum in 2 locations, pick up all orichalcum items and return them to secret lair.
The boxes for the shipment are on the south part of the map near the center at the two piers down there. make sure you break boxes until the minimap ping is gone. Some of the crates can be easy to miss on the dark map.