Sentinel

This is about the Forerunner constructs. For the UNSC AI type, see Navy Sentinel. For the achievement, see Sentinels (achievement). 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". All variants are produced by Sentinel manufacturing facilities. The Sentinels were revered by the Covenant as "holy warriors of the Sacred Rings".

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. 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.

Aggressors
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.

Assembler
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.

Automated Turret
These turrets are portable can can be deployed.

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.

Council Sentinel
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.

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.

Gatherer
Gatherers seem to be deployed exclusively on Line installations, and serve as collectors of biological specimens for the resident Monitor. They have offensive capabilities similar to Aggressors.

Protector
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.

Retriever
The Retrievers, otherwise known as the Strato-Sentinels, are massive terraforming machines, used to gather raw materials for large-scale Forerunner astroengineering projects such as the Halo Array.

Onyx Sentinel
Onyx Sentinels both guard and make up the artificial planet of Onyx. With a much more powerful individual energy beam, as well as the ability to combine and transform themselves in order to complete a specific task. Onyx Sentinels are far more formidable opponents than their more standard variants.

Safeguard
Fitted with powerful sterilization beams, the Safeguard Sentinels are highly efficient at containing and cauterizing intrusions into their assigned protectorate.

Super Sentinel
Considerably larger than Aggressors, Super Sentinels fill the same role as the Enforcers - combating large-scale Flood outbreaks.

Unidentified Sentinel variant
The largest Sentinel variant seen to date, this Sentinel is able to level entire cities with its weaponry.

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.