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, formally designated as Z-1500 Automated Systems Drones 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. As the basic type of the fairly inclusive Sentinel class, Aggressors are generally referred to simply as Sentinels.

Design detail
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 Sentinel beam. Their powerful beam weapons and defensive energy shields 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, exotic form of impulse drive which enables them to travel at high speeds. They are also capable of tapping into their installation's teleportation grid.

There are at least two variants of the Aggressor Sentinel; the most common model is grayish white in color while the specialized model is golden. The differences between the two variants lie solely on their offensive and defensive systems; the gold-colored version sports superior weaponry and shielding system to the common grayish-white model's.

Aggressors are created at specialized manufacturing facilities, massive airborne platforms that are located on every Halo installation and on other key Forerunner installations such as shield worlds. These compounds produce Sentinels when facing either a Flood outbreak or an external threat to their local installation. The massive assembly lines, at maximum performance, can produce Sentinels at a very fast rate to either supplement existing Forerunner defenses or to replace those destroyed for whatever reason.

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 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 electromagnetic pulse, a tactical last resort that disables all nearby electronics.

Changes from Halo: Combat Evolved to Halo 2

 * The Aggressor Sentinel received a graphical update.
 * There are now two variants of Aggressor Sentinel.
 * The Aggressor Sentinel's beam weapon can now be used when the Sentinel is destroyed.

Changes from Halo 3 to Halo 4

 * Sentinels have undergone a moderate visual redesign, now sporting a more rounded carapace.
 * Sentinels now make a much less shrill 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 when destroyed; instead they teleport away after sustaining severe damage. Because of this behavior, combined with the lack of the weapon's appearance in the game, the Sentinel beam cannot be obtained in Halo 4.

Trivia

 * Chris Hughes created the original concept art of the 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.

List of appearances

 * Halo: Combat Evolved
 * Halo: Combat Evolved Anniversary
 * Halo: The Flood
 * Halo 2
 * Halo: Ghosts of Onyx
 * Iris
 * Halo 3: The Cradle of Life
 * Halo 3
 * Halo Wars
 * Halo Legends
 * Origins
 * Halo: Blood Line
 * Halo: Cryptum
 * Halo: Primordium
 * Halo 4
 * Spartan Ops
 * Halo: Silentium