Aggressor Sentinel

Sentinels are ancient, Forerunner-constructed robots. They possess limited Artificial Intelligence, and are programmed to suppress Flood outbreaks and protect Forerunner technologies.

Background
Sentinels are essentially hovering drones armed with energy shields, propulsion systems, and energy weapons called Sentinel Beams. They were designed to protect Forerunner technologies, such as the Halo Array, from both outside interference and from the Flood. It is implied that prior to the Forerunners' discovery of the Flood, Sentinels (or a previous model of them) were used as assistants or repair drones; in Halo 3, simple manipulator arms can be seen on sentinels.

The mechanical nature of the Sentinels made them ideal combat units against the Flood, an enemy that could convert conventional biological infantry into biomass - by deploying the Sentinels en mass, the Forerunners could battle the parasite without contributing to its numbers. Sentinels would later serve as basic repair, maintenance and security drones for Forerunner installations and facilities where the Flood was kept, under the jurisdiction of local Monitors, working in conjunction with nearby Constructors and Enforcers. The Covenant regard Sentinels and other constructs left behind by the Forerunners as "Holy Warriors of the Sacred Rings," indicating that they have previously encountered Sentinels before they discovered the first Halo rings.

Functions
Sentinels, at one point, operated automated facilities, such as Threshold's Gas Mine, for "untold centuries". Evidence of this remains; some Sentinels have been seen repairing Covenant vessels, and others have been seen carrying Gas Containers.

Combat-enabled Sentinels are the protectors of the Halo Installations. Under the command of the local Monitor, Sentinels serving on an installation are used to contain small outbreaks of the Flood. However, they can only manage the Flood for a short time. Should an outbreak grow beyond the Sentinels' ability to control it, Enforcers and shielded Sentinel Majors are activated by the local Monitor to assist the Sentinels.

Sentinels have also been seen on the Ark and on Onyx; the Sentinels at the latter location appear to have been augmented.

Combat


The drones are equipped with a plasma-based energy weapon called a Sentinel Beam, which they fire in a sweeping motion. When not engaged in combat, Sentinels contract into smaller forms, concealing their weapon. When a Sentinel is destroyed, its weapon may still be usable(in Halo 2 and 3 only); any foot soldier can pick the beam up and fire it manually.

Plasma weapons are very effective against Sentinels. When a Sentinel is destroyed through any means, it explodes, and the blast can deplete energy shields and Jackal arm shields, stun nearby Sentinels, cause Grunts and Jackals to panic, and kill unshielded Elites and SPARTAN-IIs.

In Halo: Combat Evolved, Sentinels will give off sparks for several seconds if they take serious damage. Also, if a Sentinel gets hit by a Flood Combat Form, it makes a metal clang noise, does not take damage, but will react to the force of the Flood's jump. When a Sentinel is destroyed (in Halo 2 or later) it explodes and releases an EMP blast which will completely drain a player's shields if they are too close.

Appearances
First appearing in Halo: Combat Evolved, Sentinels have only been seen in the Halo Trilogy's Campaigns and in Halo Wars. In Halo 2, the Sentinels' weapons became usable by the player, and they remained usable in Halo 3. A non-canonical Needler Sentinel was also left in Halo 2. In Halo Wars, Sentinels serve as backup units; once the player controls a Sentinel Factory, Sentinels will start following the player's orders until the shop is destroyed or captured by the enemy.

Weaknesses
Sentinels fare poorly against Covenant plasma weapons, specifically the Plasma Pistol. The Plasma Pistol's semi-homing ability is fatal against Sentinels, as one overcharged shot can destroy a Sentinel. The Sentinel is also weak against its own Sentinel Beam. It can be surmised that these weaknesses are due to the nature of their weaponry and structure; the Sentinel constructs were built primarily as a means to control the Flood, not as a means to combat Human or Covenant weaponry. With this in mind, a few shots from the Plasma Rifle, Needler and Covenant Carbine can eliminate a Sentinel, as can other rapid-firing weapons such as the SMG and Assault Rifle. A Battle Rifle performs well against Sentinels as well.

Players can also quickly disable a Sentinel with a melee attack to the front, which will cause it to spin out of control and explode. As the Flood are very capable of melee fighting, the Sentinels are unusually weak in this regard, considering their primary function is theorized to be the effective combating of Flood outbreaks. Even a nearby explosion from a Brute Shot, grenade, or anything stronger can destroy a Sentinel easily.

Trivia

 * In Ghosts of Onyx, an Onyx Sentinel approaches Ash, one of Kurt's Spartan-IIIs, and attempts to communicate with him, first in Latin, then in English. Later in the book, Dr. Halsey confirms that this drone was a Sentinel, therefore proving that the Sentinels of Onyx have a slightly more advanced AI than those of any other Installations (besides Monitors).
 * Of interesting note, the Bestiarum, which is taken from a Forerunner perspective, gives species' names in Latin as well. So a Sentinel speaking Latin could show some sort of link between the Forerunners and the Latin language.
 * There is a lone Needler Sentinel found on the level Quarantine Zone. It fires sporadic Needler rounds and drops a Needler when destroyed. The Sentinel is non-canonical and is a development leftover.
 * Chris Hughes created a crablike machine with many animating parts that gave the Sentinel life and character.
 * It is possible to 'stick' a Sentinel with a Fragmentation Grenade. The Sentinels' in-game model is essentially hollow, so a well-thrown Frag can get stuck inside. This is easiest from in front and just above the target.
 * In Halo: Combat Evolved, Sentinels would fold into a compact "idle" form while not in combat. In Halo 3, they will fold into this form and clatter to the floor when hit with small impacts, such as brushing against a passing vehicle.
 * Normal shielded Sentinels (like the ones on the Ark, at the bridge you must activate) have relatively weak shields, as it only takes three-five shots with the Carbine to penetrate their shield and destroy them.
 * On certain levels in Halo 3, it is possible to 'surf' on Sentinels, by standing on the furthest possible point from the front of the Sentinel. This is easiest on The Ark, at the point where the light bridge must be opened for the Scorpions and Warthogs. The Sentinel will, eventually, hover over the edge, holding you up 50% of the time. Remaining on top of the Sentinel allows you to reach inaccessible locations.
 * The Bubble Shield, created by the ONI, shares many similarities to that of the shielding technology of the Sentinel, especially that of the model used at Onyx.
 * Sentinels can be seen combating Covenant troops on the Ark, prior to clearing the LZ for Forward Unto Dawn. The fight can be seen at the entrance where you flip the switch to let the Scorpions and Warthogs cross the light bridge.
 * The Forerunners referred to the Sentinels as "our very basic countermeasures."
 * In Halo Wars, Sentinels can be produced to fight for you in the map Repository.
 * In the Halo 3 multiplayer maps Construct and Isolation, they can be seen in the distance and can be shot down.
 * Sentinels share both similar design and functions to that of the P'fhor Defense Drones from the Marathon (Video Game Series). In addition, they share the same weaknesses of being unable to withstand Shotgun and Plasma Pistol/Fusion Pistol fire.
 * You can cause the message 'Hold RB to take Sentinel Beam' to appear on the Halo 3 level The Covenant. Play normally until the Arbiter has killed Truth then pick up a Plasma Pistol. Then, instead of going through the door back to the second bridge, grenade jump or hammer jump onto the roof of the structure. You should see Sentinels at the second bridge, only they will not attack the Flood; they will float in the air, idle. Kill the Flood combat forms for them. Now there will be a Sentinel floating above the bridge. Overcharge the Plasma Pistol, aim at the Sentinel, and release. This will bring the Sentinel to ground level and the message 'Hold RB to take Sentinel Beam' will appear when you approach it. Note that approaching the Sentinel while carrying a Sentinel Beam will result in no message being displayed. This seems to indicate that Bungie planned to allow the player, the Chief, to exchange weapons with a Sentinel in Halo 3. This may indicate that the Needler Sentinel of Halo 2 was not necessarily a dropped concept.