Skyrim Secondaries- Forbidden Legends: Reaching Back to Reachwater Rock

Now that you've completed all three of the dungeons of the Gauldurson brothers, it's time to head back to where you started and wrap this whole thing up.  Just a warning- you need to be either a fantastic melee combatant or toting around a powerful melee follower to do well in this last part of the quest.  Your largest obstacle here is a sequence of very difficult fights, and unless you're absolutely swimming in Health potions, you're going to get your head lopped off at some point, probably multiple times if you keep reloading the saves.

Return to Reachwater Rock through whatever method you prefer, and make your way around the edge of the main cave and up the stone bridge to the puzzle door hall.  Using the Ivory Dragon Claw, you can decipher that the correct combination is Eagle, Eagle, Dragon- with the rings set to those animals, apply the Claw and open the door, which drops into the floor as usual.

This opens a broad stone stair leading downward into the mountainside.  Keep an eye out for burial urns and gatherable alchemy reagents as you go, to the area lit by braziers.  Check carefully and you can pick up the contents of a couple more urns and a large health potion lying on a collapsed shelf.  Proceed down the rest of the stairs into the room at the bottom- but don't go too quickly or the falling porticullis could cut you off from anyone you've brought with you.

The large room you've entered here is your destination for the entirety of the quest line.  Make note of all the room's features- stairways to varying-level platforms, several coffins on the walls, a large dais at the far end with an altar on it (and a suspicious golden glow)....

When you approach the far end of the room you'll see three stands on the altar.  Use them to place the three fragments of the Gauldur Amulet, and you will activate the last fight.  The three Gauldurson brothers reappear within moments, their forms spectral and translucent- and you're going to have to beat them one at a time without any significant rest breaks.