Aggressor Sentinel

"Sentinels! The holy warriors of the sacred rings!"

- A Special Operations Sangheili exclaimed upon seeing Aggressor Sentinels in the Threshold gas mine.

Aggressor Sentinels are the most common Sentinel variant. They possess limited artificial intelligence, and are programmed to act as security and maintenance drones, suppress small-scale Flood outbreaks, and protect Forerunner technology.

Design detail
Aggressor Sentinels are grayish white in color and have two arm like appendages with two grapple fixtures at the end of each arm and a central "head," curved undercarriage containing an offensive Sentinel beam. Their powerful offensive beam weapons and defensive energy shield make them very effective in combat against intruders or foreign objects that present themselves as threats. Their smaller lasers are used for both repairing damaged Forerunner constructs in conjunction with Constructors, and self-repair should the Aggressors be damaged in combat. Aggressors are propelled by both an anti-gravity unit located on the undercarriage and small engines on the back of the unit. They are capable of traveling at high speeds but cannot sustain long range travel.

Aggressors are created at Sentinel manufacturing facilities, massive airborne platforms that are located on every Halo Installation. These facilities, located on Forerunner installations that utilize Sentinels as security and maintenance, produce Sentinels when facing either a Flood outbreak or an external threat to their local installation. These massive assembly lines, at maximum performance, can produce Aggressors at a very fast rate to either supplement existing Forerunner defenses or to replace those destroyed for whatever reason. These factories are not limited to just the Halo Array - manufacturing facilities are located on every Forerunner construct that relies on Aggressors for guarding, although these facilities are different in appearance and may be either ground-based or airborne platforms.

Operation
Built contemporaneously with the construction of the Halo Array, Aggressors were utilized as all-purpose workers and guards on Forerunner stations. Following the construction of the Halo Installations, Aggressors were reconfigured to be the first line of defense against Flood outbreaks and other external intruders. The mechanical nature of the Aggressors make them ideal combat units against an enemy that could convert conventional biological infantry into biomass - by deploying the Aggressors en masse, the Forerunners could battle the parasite without contributing to its numbers. Controlled by their own basic AI functions, as well as their Installation's Monitor, Aggressors patrol the Halo rings tirelessly and attack as their protocol demands.

When not in combat Aggressors serve as basic repair, maintenance and security drones for Forerunner installations and facilities where Flood samples are kept, working in conjunction with Constructors and Enforcers to ensure that the facility remains in working order. Aggressors are viewed by the Covenant as "Holy Warriors of the Sacred Rings", machines meant to be worshiped and respected by all who believe in the path.

Combat
The Forerunners refer to the Aggressors as "our very basic countermeasures," and are the first automated defense systems to assist a situation and generate a proper response protocol. While Aggressors are strong enough to handle small-scale battles, they are more of a supplement rather then a reliable combat system. The Aggressors' weapons systems are limited and not as powerful as some of the other Forerunner weapons employed against the Flood, and can only hold them at bay for a limited amount of time.

Because of their limited AI, they are only able to handle situations on a basic scale, which makes them vulnerable to strategic Flood attacks. Because of their limited intelligence they are not often able to perform evasive maneuvers when facing imminent attack and are usually destroyed. In overwhelming numbers, Aggressors are more than capable of handling a situation for a certain time until more permanent measures can be applied.

The Aggressor's energy shield is their primary defense mechanism. Once taken down, it is susceptible to any type of small-arms fire. When an Aggressor has been damaged beyond repair, it explodes violently and releases a powerful EMP blast. This EMP blast is a strategic last resort, as the EMP blast will disable any nearby electronics, including any weapons or shielding the enemy may be using.

Trivia

 * Chris Hughes created the original concept art of the Aggressor Sentinel.
 * There is a lone Needler Aggressor Sentinel found on the Halo 2 campaign level Quarantine Zone. It fires sporadic Needler rounds and drops a Needler when destroyed. This Aggressor is the result of a non-canonical glitch.
 * On certain levels in Halo 3, it is possible to 'surf' on Aggressor Sentinels, by standing on the furthest possible point from the front of the Aggressor. This is easiest on The Ark, at the point where the light bridge must be opened for the Scorpions and Warthogs. The Aggressor will, eventually, hover over the edge, holding the player up about half of the time. Remaining on top of the Aggressor allows the player to reach inaccessible locations.
 * Aggressor Sentinels share both similar design and functions to that of the P'fhor Defense Drones from the Marathon series. In addition, they share the same weaknesses against shotgun and plasma/fusion pistol fire.
 * In the Deliver Hope trailer, the shielding on the Covenant appears similar to the energy shields on the Aggressors in Halo 3.