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 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 footsoldier. 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, if a Sentinel gets melee'd (usually by a Flood Combat Form) it makes a metal clang noise and does nothing. The same happens in Halo 2, but with 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 melee'd will be sent flying away then exploding but with a small EMP blast if it hits something while flying away.

Despite 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
Sometimes sentinels will float along the ground.
 * Some Sentinels have active camo.
 * There is a Needler Sentinel found on the level Quarantine Zone. It fires Needler rounds and drops a Needler gun when destroyed. This is an Oversight 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.
 * 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.
 * Sentinels can be seen using the same beam as a Constructor in The Arbiter when the lift is taking you down into the first hanger.
 * You can see Sentinels on the Halo 3 maps Construct and Isolation. You can shoot down Sentinels using long range weapons.
 * 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 tanks and warthogs) by jumping on top of it. If you land on it successfully you can stay on. When you land on it one of two things will happen: #1 it will move as normal at its normal height, or #2 it will sink and float/move along touching the ground.


 * 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.

Related Links

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