Aggressor Sentinel

A Sentinel is a primordial Forerunner-constructed automated electronic construct with a limited Artificial Intelligence that is made specifically to kill and contain Flood.

Introduction
Sentinels were designed to guard the Halo Array Installations and many other Forerunner constructions, and are under the jurisdiction of the local Monitor. They are the guardians of the Halo's, and are hovering drones that are armed with energy weapons called Sentinel Beams. The Sentinels were constructed by the Forerunner for the purpose of fighting the Flood, though it is implied that prior to the Flood being discovered, they or at least a previous model of them, were used as assistants or repair drones, considering the fact that they possess simple interactable arms with which they have been seen to use (one in Halo 3 used these arms to analyze the Master Chief, before retracting them and flying off). The reason behind this was that no more Forerunner would be infected by the Flood, thus slowing down their growth and consumption. When not engaged in combat, they contract into smaller forms, concealing their weaponry and using smaller lasers to repair and manage the area around them. Some Sentinels are equipped with Energy Shields, though all Sentinels can be easily destroyed by Plasma weapons, and human weapons such as the Assault Rifle. Sentinels also appear to utilize a high-speed method of propulsion when they need to travel great distances, as witnessed by UNSC forces on the Ark.

Functions
Sentinels have been known to have been created long before Installation 04 was ever even conceived. They operated automated facilities such as Gas Mines like the one found on Threshold for "untold centuries" before the facility was redesigned to research the Flood. Therefore it is possible that they were only outfitted for combat after the Flood was discovered.

The Sentinels are the protectors of the Halo installations, and are controlled by the Monitor of the Installation. Their primary purpose is to contain small outbreaks of Flood, however they can only manage the Flood for a short time. Should an outbreak grow beyond the Sentinels' ability to control, Enforcers and shielded Sentinel Majors are activated by the Monitor of the Installation to assist the Sentinels.

On the level The Arbiter, as you enter the main hangar with the Seraph fighter, you can see one Sentinel firing a green beam at the Seraph, the same as the ones used by the Constructor drones in the level Sacred Icon. This shows that at least some Sentinels serve to repair functions. You can also see the Sentinels carrying Gas Containers.

Combat


They are equipped with an energy weapon called a Sentinel Beam which they fire in a sweeping motion in halo: combat evolved. In th other halos, it is mostly just a single beam instead of a trail behind it. The Sentinel Beam comes in two types, red and blue. The blue is more powerful than the red, though it overheats much faster. In Halo 2 and Halo 3, after a Sentinel is destroyed, these weapons can be picked up and wielded by a foot soldier. In Halo: Combat Evolved, when the sentinel is destroyed, the weapon is destroyed also. Because of this it is impossible to get a Sentinel Beam in Halo 1. When not engaged in combat, they contract into smaller forms, concealing their weapon. In later levels of Halo 2, Sentinel Majors with personal energy shields and a more powerful blue beam make their debut, though all Sentinels are easily destroyed by plasma weapons.

When a Sentinel is destroyed by any kind of explosive weapon or any weapon that can do a lot of damage in one shot it causes a blue EMP blast that has an extremely large radius, which can deplete personal energy shields, Jackal arm shields, temporarily shutdown nearby Sentinels, can cause Grunts and Jackals to cover their head with their hands and can rarely even kill an Elite or Spartan if their shields are already down.

In Halo: Combat Evolved, Sentinels will give off sparks for several seconds if they take serious damage. Also, if a Sentinel gets hit (usually by a Flood Combat Form) it makes a metal clang noise and does nothing. When destroyed, they will crash to the floor and violently explode with enough force to seriously hurt or even kill unshielded enemies. The same happens in Halo 2, but being hit with an Energy sword will cause it to explode and release an EMP blast. In Halo 3, no matter what weapon is being used, a Sentinel getting hit will send it flying a few feet away, giving off a small EMP blast when it explodes.

During the last level of Halo 3, Halo, the Sentinels fire at your Warthog and seem to push the Warthog whilst in the air, showing that not only is the beam destructive, but it also has suppressive force though it will over heat if you fire it too long.

Despite it being integrated into the Sentinel, when destroyed the player can use the Sentinel Beam in campaign, and the weapon is available in multiplayer, in both Halo 2 and Halo 3. A needler-wielding sentinel also appears in Halo 2, which was left in the game by Bungie from early on in the making of Halo 2.

Weaknesses
Sentinels fare poorly against Plasma weapons, specifically the Plasma Pistol. The Plasma Pistol's semi-homing ability is fatal against Sentinels, as one such overcharged shot will destroy it. The Sentinel is also weak against its own Sentinel Beam. Besides these, a few shots from the Plasma Rifle and Covenant Carbine can eliminate a Sentinel, as do rapid-firing weapons such as the SMG, Assault Rifle and Needler.

In Halo: CE, Sentinels can't be killed by a melee attack. In Halo 2, they are vulnerable to melee attacks, but it will not fly and will release an EMP blast when it does so, draining your shields. This also happens after a direct hit from a fuel rod cannon. In Halo 3, they are very vulnerable to a melee attack and will sent flying in the direction meeled and explode in 3 seconds, damaging those caught in the blast. It is possible to use the Sentinel as a projectile that way.

Trivia

 * Sentinels have the same name as the machines in the Matrix trilogy but they differ greatly in appearance.
 * 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 other Installations. Although Master Chief has 343 Guilty Spark to communuicate with.
 * Using the Bump-Possession cheat, Sentinels can fire any weapon they pick up.
 * Interesting to note, the Bestiarum, which is taken from a Forerunner perspective, gives species Latin names as well. So a Sentinel speaking Latin could show some sort of link between the Forerunners, and the Latin language.
 * The Sentinel Beam, along with the Fuel Rod Gun, is one of the few weapons that does not appear on any Halo 3 maps by default. These weapons, however, can be added on in Forge.
 * In Halo: Combat Evolved and Halo 2, you can destroy any unshielded Sentinel in one shot with an overcharged Plasma Pistol.
 * There is a Needler Sentinel found on the level Quarantine Zone. It fires sporadic Needler rounds and drops a Needler when destroyed. This is an oversight made by Bungie, and should not be considered part of the Halo universe.
 * In Halo: CE, the Assault Rifle has the same animation label as the Sentinel Beam, making it possible to have a Sentinel with an Assault Rifle. However, there are no actual Sentinels that have Assault Rifles in the final product of the game.
 * In Halo Custom Edition, using bump possession or a custom map to control a Sentinel, crouching activates "idle" mode.
 * 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.
 * Like other allies, Sentinels will turn on you if you destroy three or more of them. 343 Guilty Spark will also say some of the lines usually reserved for when you're dead.
 * You can see Sentinels on the Halo 3 maps Construct and Isolation. Similar to the fish and birds seen in other multiplayer maps, you can shoot down Sentinels using long range weapons. These sentinels change your cross-hair red.
 * On the Halo 2 level The Arbiter, the Elites joining the Arbiter referred to the Sentinels as 'Guardians' or 'Warriors' of the Halo Rings.
 * Normal shielded Sentinels (like the ones in the Ark at the bridge you must activate) have relatively weak shields as it only takes 3-5 shots with the Carbine to penetrate their shield and destroy them.
 * It is possible to "surf" Sentinels (this is easiest on the The Ark where you must activate the bridge for the Scorpions and Warthogs) by jumping on top of it. If you land on it successfully you can stay and hover on it. When you land on it one of two things will happen: it will move as normal at its normal height, or it will sink and float/move along touching the ground. Sometimes, Sentinels will float along the ground, making it easier to "surf". Another method is to stand on top of the Sentinel Launcher and jump on one as it comes out.
 * In Halo 3, the Shielding technology may be similar to that of the Bubble Shield, created by the ONI.
 * 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.
 * Sentinels seem to exhibit similar combat behavior as Brute and Marines, seeking cover in Bubble Shields if one is activated.
 * The Forerunners referred to the Sentinels and their beams as "our very basic countermeasures."
 * It has been confirmed that they will appear in Halo Wars indicating that the Halo Rings may actually play a part in it, but this is most likely in multiplayer.


 * For a short period in Halo 3, Sentinels are the player's allies again (a result of the Master Chief being reunited with 343 Guilty Spark). However, Guilty Spark only had command of the Sentinels on Installation 04, it is hinted that the Sentinels on the Ark are either controlled or influenced by Mendicant Bias, via several implicating messages on the terminals found on the Ark. The Sentinels on the replacement Installation 04 are most likely controlled by Guilty Spark. Since Guilty Spark was destroyed, all Sentinels on the replacement Installation 04 became hostile to anything on it. Another possibility is that Guilty Spark added the Master Chief and the Arbiter to the Sentinels targeting ledger.
 * In Halo 2, on the level "The Arbiter", if you are spotted by a Sentinel, it will make a beeping noise to alert the others.
 * In the Halo 2 level, "The Arbiter", a Sentinel can be seen using the Construction Beam on the Seraph fighter in the hangar.
 * The Sentinels in Halo 2 are only met by the Arbiter, the Master Chief does not encounter them.
 * In Halo 2, if you throw a Frag grenade below a Sentinel, it will activate idle mode to protect itself from the blast. This may also work with the Plasma Grenade.
 * The Sentinels were also used by the Forerunners in order to combat the flood, so that the Forerunners could remain outside of direct battle and remaining uninfected.
 * After the Sentinels die they drop their Sentinel Beam which you can use.
 * Unlike the Sentinels in Halo:Combat Evolved and Halo 2, the Sentinels in Halo 3 don't fall to the ground prior to exploding when destroyed.

Related Links

 * Sentinel Beam
 * Sentinel Launcher
 * Enforcer
 * Sentinel Major
 * Constructor
 * Sentinel Production Facility
 * Onyx Sentinel
 * Needler Sentinel