Halldir’s Cairn is a small tomb/cave in the southernmost part of Skyrim that was built to house the remains of the fierce Halldir. It is populated by ghost warriors, Draugr and three unlucky fugitives. There is also what looks to be a mysterious well of power that warrents further investigation.
This cave contains an iron ore vein behind the power well structure, and a vein to the left when you enter the power well room.
There is a hidden treasure room which can be unlocked by matching the Puzzle Stones to their relevant animal (snake, whale or hawk), then pulling the lever. Be wary though, because when the chest is looted, a fire trap will spring.