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 the Vault Hunters' arrival inside the Vault, it appears from a portal and must then be fought in a four-phased battle before it is defeated. After its defeat, it transforms into The Empyrean Sentinel to continue the fight.
However, they are only dropped after defeating the Empyrean Sentinel.
After completion of The Bestest Story Ever Told, the Raid Boss versions of The Sentinel and The Empyrean Sentinel (The Invincible Sentinel and The Invincible Empyrean Sentinel) can be repeatedly fought by paying five Moonstones at the four totems outside the Vault's entrance in Eleseer. The non-Raid Boss versions can be repeatedly fought by ignoring the totems.
The raid boss version simply has more health and shields, and drops nothing when defeated.
The normally blue glowing opening of the Vault becomes red when all totems have been paid, signalling that The Invincible Sentinel rather than The Sentinel will greet the Vault Hunters inside.
The Sentinel bears great resemblance to the Asuras found in Buddhist texts. Asuras are the lowest form of deity - wicked, multi-faced beings obsessed with the vices of the world.