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 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 cutting lasers. These lasers are used to control the Flood, though they can only manage them for short periods. When not engaged in combat, they contract into smaller forms, concealing their weaponry. 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.



Functions
Sentinels have been known to have been created long before Installation 04 was ever 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 "police" 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, after a Sentinel is destroyed, these weapons can be picked up and wielded by a foot soldier. 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 meleed will be sent flying away then exploding but with a small EMP blast if it hits something while flying away.

Despite its 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 Halo:Combat Evolved, you can destroy any unshielded Sentinels in one shot with an overcharged Plasma Pistol.
 * There is a 2D Ghost Sentinel near the frigate in Sandtrap that attacks and kills people on sight.
 * There is a Needler Sentinel found on the level Quarantine Zone. It fires sporadic Needler rounds and drops a Needler gun when destroyed. This is an oversight made by Bungie, and should not be considered part of the Halo universe.
 * In Halo: Combat Evolved, 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.
 * It is possible to 'stick' a Sentinel with a Frag 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.
 * 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.
 * Sentinels can be seen using the same beam as a Constructor in the level "The Arbiter" when the lift is taking you down into the first hanger. This suggests that they may be able to make basic repairs if the Constructors have been disabled or on Installations without them. (This would explain why Installation 04 is still in fairly good shape after hundreds of thousands of years)
 * 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. This, however, cannot be done on most forge games.
 * On Halo 3 multiplayer maps Construct and Isolation, when you put your crosshairs on the Sentinels, your crosshairs 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
 * It is possible to "surf" sentinels (this is easiest on the The Ark (level) 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".


 * 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.

Related Links

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