Aggressor Sentinel

A Sentinel is a primordial Forerunner-constructed automated electronic construct with limited Artificial Intelligence.

Introduction
Sentinels were designed to guard the Halo installations and many other Forerunner constructions, and are under the jurisdiction of the local Monitor. They are the guardians of Halo, and appear to be hovering drones that are armed with lasers. These lasers are used to control the Flood, though they can only manage them for short periods of time. The Sentinels were constructed by the Forerunner for the purpose of fighting the Flood. 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 seen in Halo 3. It was recently revealed in a Game Pro review of Halo Wars that they would be available as mercenary units. This has yet to be confirmed by Ensemble.



Functions
Sentinels have been known to have been created long before Installation 04 was ever even thought of. 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, the same as the ones used by the Constructor drones in the Sacred Icon level. This shows that at least some Sentinels serve repair functions. You can also see the Sentinels carrying gas canisters.

Combat


They are equipped with an energy weapon called a Sentinel Beam. 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 destroyed, there is no weapon to pick up. 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 meleed (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.

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.

Trivia

 * In Ghosts of Onyx, a drone of alien design 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.
 * In Halo: Combat Evolved, 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.
 * The final model of the Sentinel was loosely based on the concept art.
 * 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.
 * 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.
 * When a Sentinel is inactive, it will "fold" up its weapon into its body, its arms and head will move closer to its body.
 * 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.
 * On Halo 3 multiplayer maps Construct and Isolation, when you put your cross-hairs on the Sentinels, your cross-hairs turn red.
 * On the level 'The Arbiter', the Elites joining the Arbiter referred to the Sentinels as 'Guardians' or 'Warriors' of the Halos.
 * 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.
 * In the level The Ark, if you hit a Sentinel with a Brute Chopper gently in the shield wall, it will huddle in a closed/fetal position. Then in a few seconds, it will pop back up.
 * 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".
 * In Halo 3, the Shielding technology may be that of the Bubble Shield, created by the ONI.
 * In the last mission of Halo 3 (named Halo) the beam pushes your vehicle during the last part of the mission.
 * 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.
 * People have jumped on top of a Sentinel while it is still moving to avoid flood. It is called "Sentinel Surfing".
 * 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.
 * 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. 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. If you look carefully, the Sentinels may fall down with platforms on the level Halo.
 * In Halo CE, when using bump position to control a sentinel, crouching activates "idle" mode


 * It has been comfirmed that they will appear in Halo Wars indicating that the Halo Rings may actually play a part in it.

Related Links

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