Aggressor Sentinel

A Sentinel is an ancient Forerunner-constructed automated electronic construct. It has a a limited Artificial Intelligence that is made specifically to combat the Flood, should an outbreak occur.

Introduction
Sentinels were designed to guard the Halo Array Installations and many other Forerunner constructs, and are under the jurisdiction of the local Monitor. They are hovering drones armed with energy weapons called Sentinel Beams. The Sentinels were constructed by the Forerunners for the purpose of combating the Flood. It is implied that prior to the Flood being discovered, they, or a previous model of them, were used as assistants or repair drones, considering the fact that they possess simple retractable arms with which they have been seen in Halo 3 to analyze the Master Chief, before retracting them and flying off. The Sentinels are used as combatants, because they are invulnerable to Flood infection, slowing down the parasite's growth and consumption. Some Sentinels are equipped with Energy Shields, though all Sentinels can be easily destroyed by Plasma weapons, and with slightly less efficiency 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, at one point, 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.

On the level The Arbiter, as you enter the main hangar with the Seraph fighter, you can spot 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 would seem to indicate that some Sentinels serve as repair drones as well. Sentinels can also be seen carrying Gas Containers.

The later, combat enabled 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 local monitor to assist the Sentinels.

Weaknesses
Sentinels fare poorly against Covenant 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, Needler and Covenant Carbine can eliminate a Sentinel, as do other rapid-firing weapons such as the SMG and Assault Rifle. You can also quickly disable a Sentinel by melee attack to the front, which will cause it to spin out of control and explode.

Trivia

 * You can "assassinate" Sentinels. The easiest way to do this is on The Maw, at the beginning of the level when you hide in the hall right to the lifeboat airlock. If you wait for a while, 4 Sentinels will float in, facing the other way, and you can melee them from behind.
 * 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 any other Installations (besides Monitors).
 * 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.
 * There is a lone 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.
 * 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 in 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.
 * Normal shielded Sentinels (like the ones on 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.
 * The Bubble Shield, created by the ONI, shares many similarities to that of the shielding technology of the Sentinel, especially that of the model used at Onyx.
 * 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.
 * The Forerunners referred to the Sentinels as "our very basic countermeasures."
 * 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.
 * It is unknown why the Onyx Sentinels are so much more powerful than those found on the Halo Installations. Perhaps for game play reasons, Bungie probably didn't want the player to be playing hide and seek with Sentinels for most of the game. Another reason would be because of the importance of the Onyx Shield World, since it would, according to plan, be harboring millions of lives, they most likely designed the Onyx Sentinels to be more powerful and damage resilient than those found on the Halo Array because, if the Flood somehow survived, the Forerunners would be in dire need of extra protection.
 * In the Halo 2 level The Arbiter, the Sangheili accompanying the Arbiter referred to the Sentinels as 'Warriors of the Sacred Rings', this is appropriate due to the design and function of the Sentinels is to combat anything that threatens the safety and/or functions of the Halo Arrays.
 * The only map in Halo Wars where you can buy sentinels from a Sentinel Shop is Repository.

Related Pages

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