Sentinels are autonomous machines created by the Forerunners, designed for the construction, maintenance and defense of their structures and facilities. 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 shops, or Sentinel launchers. The Sentinels were revered by the Covenant as "holy warriors of the Sacred Rings", 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. 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.
- 1 Types of Sentinels
- 1.1 Aggressors
- 1.2 Assembler
- 1.3 Automated Turret
- 1.4 Constructor
- 1.5 Controller
- 1.6 Council
- 1.7 Enforcer
- 1.8 Gatherer
- 1.9 Protector
- 1.10 Strato
- 1.11 Onyx
- 1.12 Safeguard
- 1.13 Super
- 1.14 Unidentified Sentinel variant
- 1.15 Sentinel Defense Network
- 2 Promethean Types
- 3 Behind the scenes
- 4 List of appearances
- 5 Sources
Types of Sentinels
- Main article: 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. 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.
- Main article: Assembler Sentinel
Assemblers are presumably used to build and maintain Forerunner installations and technology, like the similarly named Constructors. Though they are not designed for combat, they will suspend their primary duties and engage Flood forms in the event of an outbreak.
- Main article: Forerunner automated turret
These turrets are portable and can be deployed.
- Main article: 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.
- Main article: 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.
- Main article: Enforcer
The Enforcers are equipped with a variety of offensive and defensive measures, including pulse beams, missiles, a last-resort Sentinel beam, and three frontal energy shields. They are designed to handle major Flood outbreaks, but are also capable of defending their Installation from other external threats.
- Main article: Gatherer
- Main article: Protector Sentinel
Shield World 0459 (Etran Harborage)
- Main article: Protector Sentinel (Shield World 0459)
Roughly similar in appearance to regular Aggressors, Protectors are modular units capable of being modified into one of three variants: Offensive, which is a weaker version of the standard Aggressor; Shield, which projects an energy shield around its host unit; and Healing, which provides medical assistance to its host after combat.
Installation 00 (The Ark)
- Main article: Protector Sentinel (Installation 00)
These types of Protector Sentinel were deployed on Installation 00, also known as the Ark.
- Main article: Strato-Sentinel
- Main article: Retriever Sentinel
The Retrievers are a Strato-Sentinel type.
- Main article: Steward Sentinel
- Main article: 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.
Fitted with powerful sterilization beams, the Safeguard Sentinels are highly efficient at containing and cauterizing intrusions into their assigned protectorate.
- Main article: Super Sentinel
Considerably larger than Aggressors, Super Sentinels fill the same role as the Enforcers - combating large-scale Flood outbreaks.
Unidentified Sentinel variant
- Main article: Unidentified Sentinel variant
The largest Sentinel variant seen to date, this Sentinel is able to level entire cities with its weaponry.
Sentinel Defense Network
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. 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. 
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.
- Main article: 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.
- Main article: Z-2500 autosentry
- Main article: Z-510 focus turret
This turret charges up and fires a powerful beam.
Behind the scenes
List of appearances
- Halo: The Essential Visual Guide, page 167
- Halo 2, campaign level The Arbiter - "A Whisper in the Storm"
- Halo: Hunters in the Dark, page 83 (Google Play edition)
- Halo Wars 2, Phoenix Logs — Forerunner Sentinels
- Halo Wars 2, campaign level Under The Dark
- Halo Wars 2
- Halo: Cryptum, page 294
- Halo: Blood Line
- 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."
- Halo Waypoint - Canon Fodder: Bounty Hunters
- Halo Wars 2, Campaign level The Archive
- Halo Wars 2, Campaign level The Halo
- Halo Wars 2, Campaign level Under the Dark
- Halo 4 Official Site: CHARACTERS
- Halo Infinite reveal trailer
- YouTube - The Making of the Halo Infinite Announce Trailer