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. 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 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 weaponry. 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.

The Sentinel Beam is also available in Halo 2's multiplayer. It can be found inside the bases on Backwash and Desolation.

Trivia

 * 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.
 * 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 hanger.
 * You can see Sentinels on the Halo 3 maps Construct and Isolation.


 * 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