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, more commonly known as Sentinels and referred by the Covenant as "Holy Warriors of the Sacred Rings" , are Forerunner automatons programmed to act as security and maintenance drones, suppress small-scale Flood outbreaks, and protect Forerunner installations.

Design detail
Aggressors 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 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 also capable of traveling at high speeds.

There are at least two variants of Aggressor Sentinels; the most common model being grayish white in color while the specialized model being gold in color. The differences between the two variants lie solely on their offensive and defensive systems; the specialized-gold sports superior weaponry and shielding system than the common grayish-white model.

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
Aggressors were utilized as all-purpose workers and guards on Forerunner stations. Following the construction of the Halo Array, they 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 installation's vast terrain and network tunnels 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.

Combat behavior
Aggressors are the first of Forerunner 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, a strategic last resort that disables all nearby electronics.

Changes from Halo 3 to Halo 4

 * Sentinels have undergone a moderate visual redesign.
 * Sentinels now make a distinctive whirring sound while flying.
 * Sentinels now fire a bluish-colored beam, similar to the beam of the Sentinel Majors of Halo 2, instead of an orange beam.
 * Sentinels will no longer explode into pieces when destroyed and will instead "warp out" and disappear. Because of this odd behavior, combined with the lack of the weapon's appearance in the game, the Sentinel Beam is now impossible to obtain.

Trivia

 * Chris Hughes created the original concept art of the Aggressor Sentinel.
 * A lone Aggressor that fires sporadic Needler rounds can be found on the Halo 2 campaign level, Quarantine Zone. The Aggressor will drop a Needler when destroyed. This is the result of developer experimentation with giving Sentinels different weapons.
 * 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.
 * Sentinel Aggressors appear in Halo 4 on the levels Requiem, Reclaimer, Midnight and Spartan Ops Chapter 9 and do not attack the player, and even if they are attacked, they will only vanish through portals. On Reclaimer, if not attacked, they will ignore the player but will fly towards the back of a symmetrical hall, like they were attempting to lead the player towards something. While they will not engage the player, they will, however, lend aid in assistance in attacking Covenant remnant and Promethean forces. No Sentinel Beam will be dropped off by destroyed Sentinels however.

List of appearances

 * Halo: Combat Evolved
 * Halo: Combat Evolved Anniversary
 * Halo: The Flood
 * Halo 2
 * Halo: Ghosts of Onyx
 * Halo 3
 * Halo 3: The Cradle of Life
 * Halo: Blood Line
 * Halo Wars
 * Halo Legends
 * Origins
 * Halo 4