Sentinel

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

Sentinel Aggressor
The Sentinel 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.

Sentinel Major
Sentinel Majors are larger, more powerful Sentinel models used for heavy combat situations. They provide backup for Enforcers in the event of a massive Flood outbreak or external attacks on a Halo Installation.

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
Essentially heavy Aggressors, 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 Halo Installation from other external threats.

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.

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

Protector Sentinel
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.

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

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 exclusively deployed 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.

Constructor
Constructors are specialized Sentinels, designed solely for the construction and servicing of Forerunner structures. It was these Sentinels that built and maintain the Sentinel Wall on Installation 05.