Guardians are the protectors of The Vault in Borderlands and The Vault of the Sentinel in Borderlands: The Pre-Sequel. They have alien body designs, use alien weapons technology and use extremely tough shields to protect their fairly weak bodies underneath. Primarily encountered late in both games, guardians will often be found fighting against the forces of the Crimson Lance in Borderlands and alongside the Lost Legion in Borderlands: The Pre-Sequel.
The guardians are beings left on Pandora and Elpis by the Eridian race to protect important Eridian artifacts and locations. One such location is the mine underneath the Dahl Headlands, in which three guardians (one spectral and two wraiths) have been left as protectors for an Eridian Artifact. Another, more prominent locations are The Descent and Eridian Promontory where the guardians have been set up in defensive positions covering the entire approach to The Vault.
The guardians of Elpis revealed the secrets of the Vault of Sentinel to Colonel Zarpedon and her Lost Legion, and fight alongside them to keep it a secret in Vorago Solitude, Outfall Pumping Station, Tycho's Ribs and Eleseer.
Guardians seem to be robotic in nature and are made in image of the Eridians themselves.
Shock damage is very useful for stripping away a guardian's powerful shield. Once the shield has been depleted, the guardian itself is not particularly robust and will succumb quickly to subsequent damage. Eridian weapons work just as well as shock.
The unusual pattern, speed of their movement and slender bodies can make guardians difficult to target effectively, so using an automatic weapon or a shotgun can be useful in many cases.
Their heads hang in front of their chests on long, lanky necks and are quite small, making it difficult to achieve critical hits from a distance. However, with the position of their heads, it can be quite easy to score accidental critical hits from body shots when the guardian faces towards a shooter. The region to aim for is the area where wings are connected to the body, or the upper back.
In Borderlands the Hunter talent Trespass can be the bane of the guardians once it has been advanced to its maximum level. It will allow bullets to bypass a guardian's formidable shield to apply the weapon's full force to the relatively low life of the guardian.
Most guardian types can turn semi-cloaked while approaching their targets. Although still visible they can blend into the background, and in the heat of the battle sneak up on unaware Vault Hunters.
Guardian Types (Borderlands)
The Dahl Headlands
The Descent and Eridian Promontory
Guardian Types (Borderlands: The Pre-Sequel)
- Guardians are hostile to Crimson Lance units, and when they appear together in Crimson Enclave, The Descent and Eridian Promontory, they will fight each other. This can be used tactically by lending assistance to the weaker side, or players can alternatively watch for opportunities to snatch a kill in order to gain the substantial experience rewards.
- Guardians grant very high XP rewards in comparison to their difficulty, making them ideal to use for experience point farming.
- If the guardians that use melee attacks are unable to reach their target, they will start firing plasma bolts instead until a target becomes accessible.
- The origins of the guardians were open to speculation from their first appearance in Borderlands (see the talk page). However, Borderlands: The Pre-Sequel's The Holodome Onslaught DLC gave some insight, with Athena confirming that the guardians fought throughout Borderlands: The Pre-Sequel "are biomechanical constructs -- but not flesh and blood.".
- The Borderlands strategy guide mentioned Scooter's theory about the origin of guardians: they're cyborgs sent back from a dystopian future to impregnate his mother, thus creating offspring capable of preventing the horrible future they came from.
- The names of various guardians make reference to the Christian Hierarchy of Angels but with reversed significance:
- Sera — Seraphim
- Cheru — Cherubim
- Opha — Ophanim
- Dominant — Dominions
- Virtuous — Virtues
- Principal — Principalities
- Arch — Archangel
- Putti — Putto, which isn't an angel, but an image of a child similar to Cupid in Renaissance and Baroque art.
- Guardians appear in the fifth episode of Tales from the Borderlands — The Vault of the Traveler, where they guard The Traveler's teleportation gland.
- Concept artist Keith Thompson is responsible for Eridian Guardian and Eridian Weapon designs.
- For Borderlands he made the concept art for both Sera Guardians and Arch Guardians, with the game only using the Arch Guardian design, with various wings and other attachments added to differentiate the various types. Borderlands: The Pre-Sequel uses new models based on both of these designs, using a similar approach to differentiate each type from one another.
- The description for Eridian Guardians from Thompson's official site: "Ancient alien robots left behind to administer and guard abandoned cities. Faces reform to approximately mimic the appearance of whatever species it interacts with." 
- In Borderlands, guardians bleed blue blood. In Borderlands: The Pre-Sequel, when Nvidia PhysX is enabled, they bleed purple liquid similar to slag from Borderlands 2.
- In the game files for Borderlands are unused models and animations for a "Cherub" Guardian. The similarities in appearance reflects that these unused Cherubs eventually became Putti in Borderlands: The Pre-Sequel.