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. Once you have defeated Doomgrip, the quest item Heart of the Mountain spawns in the back of the Vault.