This quest is the last part of a quest line to restore Gauldurs Amulet. The other three parts are dungeons where you will find fragments of the amulet. You can find those guides here:
Saarthal (coming tomorrow)
You can find Reachwater Rock south-east-east of Markarth and south-south-west of Solitude:
When you enter the main cave if you jump into the water below and swim around a little you will find a chest with some loot on the bottom of the lake. You will have to pick up the claw that is on the pillar next to the dead body. The note underneath is of no importance, it just tells you that it is pretty dangerous ahead, which it is. You will to open two of the doors. I had the second claw already but I am not sure were I got it from. I am pretty sure that I would have found it in an earlier part of this same quest, presumable in Fulgunthur.
Killing Mikrul Gauldurson
Next you will face all three bosses of the previous dungeons again with the same abilities. The first spawns a number of draugr. You can kill those easily because they don’t have a lot of health. Whilst doing so be sure to stay out of Mikrul’s reach to avoid taking unnecessary damage.
Killing Sigdis Gauldurson
The second is Sigdis Gauldursun. He will spawn three copies of himself around higher positions around the room. You will just have to find the real one by hitting every one of them. The fake one’s will die instantly. Once you have dealt a little damage to Sigdis just look around the room until you find Sigdis based on his health damage. If you find that you are taking a lot of damage be sure to kill all the fake Sigdises.
Killing Jyrik Gauldurson
This last enemy is the easiest to kill. He isn’t supposed to have any friends/copies of himself helping him, but my game bugged out and I had more Sigdis copies to deal with. Jyrik is a standard boss with a lot of health. I like to tank these types of enemies and retreat when I am out of health. Be sure to move around the place so that Jyrik cannot follow you easily. He will jump off ledges, but he can only climb stairs.
After killing Jyrik a figure will appear that will reforge Gauldor’s Amulet. It is a very good Amulet, increasing your stamina, health and magicka by 30 points. Unfortunately you cannot dis-enchant this Amulet. Be sure to loot the chest on your way out. A stone will have moved straight in front of the exit of the chamber that you fought the Gauldursons in.
Full Walk Through YouTube Video: