This quest is not available in game.
Bring the Heart of the Mountain to Maxwort Uberglint in the Burning Steppes.
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 Lower Vault of Blackrock Depths, 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!