|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
Appears to be a rather complex keycard. It should be, as it detonates a small nuclear device.
Given to the Initiate by Patty, when she's caught by Attis as a blob in the mutated Secret Vault - Laboratories. It opens the final mutated section of the laboratories and starts the countdown for the vault to be destroyed by a nuclear explosion.