How do you complete forbidden legend in Skyrim in reachwater rock?

To complete the Forbidden Legend quest in Skyrim at Reachwater Rock, you first need to unlock the entrance by finding the two missing pieces of the Gauldur Amulet. After entering, navigate through the cave, defeating enemies like draugr and solving puzzles. Look for three sarcophagi, each containing a Gauldur brother. Defeat them to collect their amulet fragments, and once you have all three, return to the quest giver to complete the quest and receive the Gauldur Amulet.