Relic Coffer Key

The Relic Coffer Key is required to open the Relic Coffer Doors in Blackrock Depths. These doors cannot be lockpicked.

Source
This item is a common drop from Trash Mobs in Blackrock Depths.

Note
You need 12 Relic Coffer Keys to open all the coffer doors to spawn Watchman Doomgrip for the quest The Heart of the Mountain (quest). Once you have defeated Doomgrip, the Heart of the Mountain spawns in the back of the Vault.