Sentinel

Sentinels are semi-autonomous machines created by the Forerunners, designed for the construction, maintenance and defense of their structures and facilities. Numerous Sentinel variants exist, each tailored to a specific task.
== Origin ==
Following the construction of the Halo Rings, Sentinels were built to serve as the first line of defense against Flood outbreak and external intrusion. The non-organic composition of the Sentinels allowed the Forerunners to battle the parasite without contributing to its numbers. Controlled by their own autonomous AI and coordinated by their Installation's Monitor, Sentinels patrol the Halo rings and destroy anything deemed to be a threat.
== Variants ==
=== Aggressor ===
The Aggressor is the most common variant of Sentinel, designed for the protection of Forerunner constructs and the small-scale suppression of Flood forms. Aggressor Sentinels have two arm-like appendages, with two fine manipulators at the end of each arm. Their central head rests above a curved undercarriage housing an offensive particle lance. Their powerful beam weapons and defensive energy shields make them very effective in combat against intruders or foreign objects that present themselves as threats.
=== Enforcer ===
The Enforcer is a heavy combat unit equipped with a variety of offensive and defensive measures, including dual pulse beam cannons, disintegrator missile launchers, a particle lance, 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.
=== Annihilator ===
The Annihilator is a large, powerful Sentinel variant that is equipped with a beam weapon capable of vaporizing large amounts of Flood biomass with a single strike.
=== Constructor ===
The Constructor is a specialized Sentinel, designed solely for the construction and maintenance of Forerunner structures. They are not designed for combat, but they will engage the Flood if necessary.