Cannot pickup Gauldur amulet fragment in Geirmund's Hall

You can type player.additem 0002d753 1 to add the Gauldur Amulet fragment into your inventory. If your quest doesn't advance by adding the item to your inventory, you will need to advance the quest to next stage with the command setstage dunGauldursonQST 8.

Forbidden Legend Quest Details and Bug fixes

I passed Geirmund's Hall before the others and this quest got bugged. This is how I completed it:

1. After having the other two fragments (1 and 3) returned to Geirmund's Hall and at the place of the marker typed to console:

   player.additem 0002d753 1
   

   This got me the second amulet fragment (no progress in the Journal, though).

   setstage dunGauldursonQST 9
   

   (This had no visible result, so I'm not sure if it was necessary.)

2. Went to Reachwater Rock (cave under a waterfall south of Karthspire).

   After opening a claw-locked door inside the quest suddenly advanced to 'Forge the amulet' or something like that. There weren't any problems till the end.

It seems that a bug can occur (UESP wiki Forbidden Legend article talk page) if you've cleared the Geirmund's Hall dungeon or killed Sigdis Gauldurson (the boss in that dungeon) before starting the Forbidden Legend quest. If you do this, you might be unable to pick up Sigdis' Gauldur Amulet Fragment.

For platforms that can't use mods or console commands, such as Xbox 360 and PS3, to fix this you'd need to load a save where you haven't killed Sigdis Gauldurson yet nor started the Forbidden Legend quest. Then, start the quest before killing Sigdis Gauldurson.