Obtain the Heart of the Mountain.
For years I have sought a certain gem. It is called the Heart of the Mountain and it's the size of your fist! The Dark Iron dwarves have it locked in their vault and, try as I may, they won't let me purchase it.
So I must resort to force.
Fight your way to The Black Vault near the Hall of Crafting, breach its secret safe and gain the Heart. To do that, you must defeat Watchman Doomgrip, and he won't appear until you've opened every relic coffer in the vault!