The Sentinel is an ancient, being created by the Eridians to guard the Vault of Elpis. It appears as a humanoid, multi-faced warrior with a body seemingly carved from stone.
The Sentinel is encountered during the mission The Beginning of the End. Little information about the Sentinel is presented until Jack's Vault Hunters enter the Vault and encounter it themselves. Upon arrival inside the Vault, it appears from a portal and must then be defeated twice. First in its humanoid form, then in a gigantic form called The Empyrean Sentinel.