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This is about the Forerunner constructs. For the UNSC AI type, see Navy Sentinel. For the achievement, see Sentinels (achievement).
An Enforcer flanked by two Aggressors.

Sentinels are machine-servants[1] created by the Forerunners, designed for the construction, maintenance and defense of their structures and facilities.[2] Various Sentinel variants exist, each tailored to a specific task; because of its ubiquity, the Aggressor variant is often referred to simply as the "Sentinel". Most variants are produced by Sentinel manufacturing facilities, Sentinel factories, or Sentinel launchers. The Sentinels were revered by the Covenant as "holy warriors of the Sacred Rings",[3][4] despite engaging them in combat on various occasions.


The Sentinels follow the command of the monitor or other ancilla that is in charge of the facility they are assigned to, but even without central direction, they are able to respond to threats against their assigned installations with remarkable coordination and effectiveness.[5][6] On Installation 00, a sentinel defense network is used to coordinate the Sentinels on the Ark despite the monitor no longer being present. Sentinels follow their directives even if this entails engaging other Forerunner machines. For example, the Aggressor Sentinels of Requiem remained loyal to the Librarian against the Didact's Prometheans in 2557. Another instance is when the Constructors of Genesis remained loyal to Monitor Exuberant Witness against the Created's Promethean forces in 2558.

Sentinel Defense Network[edit]

A Sentinel Defense Network is a system that coordinates sentinel defenses to perform their duties in defense of an installation. All Installation has the Defense Network installed and each has its own protocol. Without the defense network activated, Sentinels are unable to perform their duties. This, however, does not affect the stationary defenses, which remain active even if the defense network is shut down, including containment shields.[7] The Defense Network itself is separated to local and global defense network. The local defense network only controls sentinels surrounding terminals, while the global defense network controls defenses on a global scale. The Defense Network is linked with every defensive protocol on the installation. Containment protocol on Installation 00 is linked with the Defense Network and Sentinels cannot conduct the protocol if the network is disabled.

Installation 09 heightened the security protocol and immediately launched a Retriever Sentinel to cleanse the humans and the Banished present on the rings as soon as it left Installation 00. Professor Anders successfully hijacked the local network through the control tower to temporarily take control of Retriever Sentinel to disable barriers around the control room.[8]

The Smart AI Isabel manipulated the Sentinel Defense Network to her own ends by hijacking Enduring Conviction and firing its energy projector on the Ark, causing Aggressor Sentinels to swarm on the ship and bisect it.[9]

When Voridus shut down the Sentinel Defense Network, all Sentinel activities are ceased immediately, giving Voridus a clear path to the High Charity without any opposition.[10] When Voridus reactivated the Sentinel Defense Network, the Sentinels were immediately resumed their duty, allowing Pavium and Voridus to call in Retriever Sentinel to defeat Proto-Gravemind.[11]


  • Aggressor Sentinel - The Aggressor is the most common form of Sentinel, designed for the protection of Forerunner constructs and the small-scale suppression of Flood forms.[2] There are many forms of Aggressor, though most follow the same basic layout. In the event of a severe threat, an advanced version of the Aggressor would be dispatched to assist the more common Aggressors.
  • Constructor - The Constructors are specialized Sentinels, designed solely for the construction and servicing of Forerunner structures. Like Assemblers, they are not designed for combat, but they will engage the Flood if necessary.[2]
  • Controller - These sentinels were typically used to defend valuable technology on Installation 00. Their typical weaponry is a supercharged sentinel pulse as their main weapon, with a Sentinel beam to cover their blind spot. For defensive purposes, they can create an energy shield on their forward side. This shield is significantly stronger than that of the Enforcer.[12]
  • Council - A special variant of Sentinel was used aboard the Forerunner Capital for security purposes. They possessed black armor, matching the black-and-red armor of Ecumene Council security personnel.[13]
  • Enforcer - The Enforcers are equipped with a variety of offensive and defensive measures, including shard cannons, missiles, a last-resort Sentinel beam, and three frontal energy shields.[2] They are designed to handle major Flood outbreaks, but are also capable of defending their Installation from other external threats.
  • Gatherer - Multipurpose Sentinels with the functions of maintaining ecological stability in constructed biomes and the gathering of various living specimens for Lifeworker activities.[15][16]
  • Monitor - A general purpose Sentinel, containing a high-level ancilla.[18]
  • Promethean Watcher - The Watcher is a Sentinel variant used by Promethean Knights for combat support. They can heal and augment their damaged allies and return incoming ordnance such as grenades using a gravity-displacement beam.[19]
  • Protector - A type of Sentinel encompassing a few different models.
  • Onyx Sentinel - Onyx Sentinels both guard and make up the artificial planet of Onyx. They use a much more powerful individual energy beam than the Aggressor, and have the ability to combine and transform themselves in order to complete a specific task. Onyx Sentinels are far more formidable opponents than more widely-used "standard" variants.
  • Safeguard Sentinel - Fitted with powerful sterilization beams, the Safeguard Sentinels are highly efficient at containing and cauterizing intrusions into their assigned protectorate.[22]
  • Super Sentinel - Considerably larger than Aggressors, Super Sentinels fill the same role as the Enforcers - combating large-scale Flood outbreaks.
  • Unidentified sentinel variants

Production notes[edit]

Sentinels were seen in Halo Infinite's announcement trailer at E3 2018.[23][24]


Concept art[edit]


List of appearances[edit]


  1. ^ Halo: Warfleet – An Illustrated Guide to the Spacecraft of Halo - Glossary, page 92-93
  2. ^ a b c d e f Halo: The Essential Visual Guide, page 167
  3. ^ Halo 2, campaign level The Arbiter - "A Whisper in the Storm"
  4. ^ Halo: Hunters in the Dark, page 83 (Google Play edition)
  5. ^ Halo Wars 2, Phoenix Logs — Forerunner Sentinels
  6. ^ Halo Wars 2, campaign level Under The Dark
  7. ^ Halo Wars 2, Campaign level The Archive
  8. ^ Halo Wars 2, Campaign level The Halo
  9. ^ Halo Wars 2, Campaign level Under the Dark
  10. ^ Halo Wars 2, Campaign level What Could Go Wrong?
  11. ^ Halo Wars 2, Campaign level Manifestation
  12. ^ Halo Wars 2
  13. ^ Halo: Cryptum, page 294
  14. ^ Halo Encyclopedia (2022 edition), page 329
  15. ^ Halo: Blood Line
  16. ^ Halo Encyclopedia (2022 edition), page 328
  17. ^ Halo 3, multiplayer map Epitaph
  18. ^ Halo: Warfleet – An Illustrated Guide to the Spacecraft of Halo - Glossary, page 90-91
  19. ^ Halo 4 Official Site: CHARACTERS
  20. ^ Halo Encyclopedia (2022 edition), page 327
  21. ^ Canon Fodder: Outpost Discoveries "Stewards: These Sentinels accompany other Forerunner machines and assist them with their tasks, using a combination of support systems and offensive weaponry.The largest constructs are classified as strato-sentinels, taking the roles of grand builders and overseeing architects for major Forerunner works. Those used in military applications were often classified as weapon-ships, capable of being fitted with naval-scale ordnance and defensive systems."
  22. ^ Halo Waypoint - Canon Fodder: Bounty Hunters
  23. ^ Halo Infinite reveal trailer
  24. ^ YouTube - The Making of the Halo Infinite Announce Trailer