Sentinel

Sentinel is a Forerunner term that refers to a class of autonomous machines designed for the construction, maintenance and protection of their structures and facilities. Various Sentinel variants exist, each tailored to a specific task.

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, known as the Sentinel Major, 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.

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.

Enforcer
The Enforcers are equipped with a variety of offensive and defensive weaponry, including heavy needlers, missiles, Sentinel beams, and dual energy shields covering both their top and bottom halves. They are designed to handle major Flood outbreaks but are also capable of defending their installation from other external threats.

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.

Watcher
Considerably larger than normal Sentinels, Watchers fill the same role of combating external threats.

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.

Onyx Sentinel
Onyx Sentinels both guard and make up the shield world of Onyx. With a much more powerful individual energy beam, as well as the ability to combine themselves in order to increase their overall beam power, Onyx Sentinels are far more formidable opponents.

Guardian Sentinel
Little is known about this variant's specific purpose. The Guardian Sentinel has a single red "eye", which fires a significantly more powerful beam than the standard Aggressor.

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.

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