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The Vault Monsters were creatures locked away within the Eridian Vaults, acting as both their guardians and their captives. It was originally believed that a Vault contained infinite power, wealth or other riches and while they did indeed contain remnants of technology and even weapons, the true prize lied with the power of the beasts contained within as explained by Troy and Tyreen Calypso.
Purpose of Vaults
The true purpose of the Vaults have yet to be explained by those who made them, the Eridians. However it is noted by the ancient Siren, Nyriad, that the Eridians were far more intelligent than they knew what to do with, thus they sought out creatures and questions better left unanswered. This is what lead to the discovery of The Destroyer and ultimately the fall of their civilization.
It is therefore theorized that the Vaults were created to house the beings either discovered by, or created by the Eridians as a means to prevent destruction.
Known Vault Monsters
Throughout the Borderlands games, the numerous Vault Hunters discover and defeat several of the beasts contained within these Vaults.-
The Destroyer is a massive, intergalactic menace being from another dimension, sealed within the Great Vault, capable of devouring stars,entire planets and civilizations. It appears as a gargantuan, one-eyed beast with a massive mouth and several tentacles. The Eridians, at the cost of their entire civilization, imprisoned The Destroyer within the Great Vault thousands of years ago, although a ‘false Vault’ was made to lure unsuspecting beings to sate the Destroyer’s hunger.
The Warrior is a biological Eridian superweapon lying dormant on Pandora, acting as the planet’s guardian. It resides within a Vault in the northern reaches of the planet, and can be controlled through the use of the Vault Key.
The Empyrean Sentinel
The Empyrean Sentinel was sealed within a vault on Pandora's moon, Elpis, protecting ancient and future knowledge. It is an ancient, colossal humanoid with a multi-faced head created by the Eridians. Only its upper body is seen, the torso is hidden by the floor.