Aggressor Sentinel

Sentinels are ancient, Forerunner-constructed robots. They possess limited Artificial Intelligence, and are programmed to suppress Flood outbreaks and protect Forerunner technologies.

Background
Sentinels are essentially hovering drones armed with energy shields, propulsion systems, and energy weapons called Sentinel Beams. They were designed to protect Forerunner technologies, such as the Halo Array, from both outside interference and from the Flood. It is implied that prior to the Forerunners' discovery of the Flood, Sentinels (or a previous model of them) were used as assistants or repair drones; in Halo 3, simple manipulator arms can be seen on sentinels.

The mechanical nature of the Sentinels made them ideal combat units against the Flood, an enemy that could convert conventional biological infantry into biomass - by deploying the Sentinels en massa, the Forerunners could battle the parasite without contributing to its numbers. Sentinels would later serve as basic repair, maintenance and security drones for Forerunner installations and facilities where the Flood was kept, under the jurisdiction of local Monitors, working in conjunction with nearby Constructors and Enforcers. The Covenant regard Sentinels and other constructs left behind by the Forerunners as "Holy Warriors of the Sacred Rings," indicating that they have previously encountered Sentinels before they discovered the first Halo rings.

Functions
Sentinels, at one point, operated automated facilities, such as Threshold's Gas Mine, for "untold centuries". Evidence of this remains; some Sentinels have been seen repairing Covenant vessels, and others have been seen carrying Gas Containers.

Combat enabled Sentinels are the protectors of the Halo Installations. Under the command of the local Monitor, Sentinels in an installation are used to contain small outbreaks of the Flood. However, they can only manage the Flood for a short time. Should an outbreak grow beyond the Sentinels' ability to control it, Enforcers and shielded Sentinel Majors are activated by the local Monitor to assist the Sentinels.

Sentinels have also been seen on the Ark and on Onyx; the Sentinels at the latter location appear to have been augmented.

Combat


The drones are equipped with a plasma-based energy weapon called a Sentinel Beam, which they fire in a sweeping motion. When not engaged in combat, Sentinels contract into smaller forms, concealing their weapon. When a Sentinel is destroyed, its weapon may still be usable ; any foot soldier can pick the beam up and fire it manually.

Plasma weapons are very effective against Sentinels. When a Sentinel is destroyed through any means, it explodes; the blast can deplete energy shields and Jackal arm shields; stun nearby Sentinels; cause Grunts and Jackals to panic; and kill unshielded Elites and SPARTAN-IIs.

In Halo: Combat Evolved, Sentinels will give off sparks for several seconds if they take serious damage. Also, if a Sentinel gets hit by a Flood Combat Form, it makes a metal clang noise and does nothing. When a Sentinel is destroyed (in Halo 2 or later) it explodes and releases an EMP blast which will completely drain a player's shields if they are too close.

Appearances
First appearing in Halo: Combat Evolved, Sentinels have only been seen in the Halo Trilogy's Campaigns and in Halo Wars. In Halo 2, the Sentinels' weapons became usable by the player, and they remained usable in Halo 3. A non-canonical Needler Sentinel was also left in Halo 2. In Halo Wars, Sentinels serve as backup units; once the player controls a Sentinel Factory, Sentinels will start following the player's orders until the shop is destroyed or captured by the enemy.

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 overcharged shot can destroy a Sentinel. The Sentinel is also weak against its own Sentinel Beam. It can be surmised that these weaknesses are due to the nature of their weaponry and structure; the Sentinel constructs were built primarily as a means to control the Flood, not as a means to combat Human or Covenant weaponry. With this in mind, a few shots from the Plasma Rifle, Needler and Covenant Carbine can eliminate a Sentinel, as can other rapid-firing weapons such as the SMG and Assault Rifle.

Players can also quickly disable a Sentinel with a melee attack to the front, which will cause it to spin out of control and explode. As the Flood are very capable of melee fighting, the Sentinels are also weak in this regard. Even a nearby explosion from, say, a Brute Shot, grenade, or anything stronger than those mentioned can destroy a Sentinel easily. A Battle Rifle performs well against Sentinels as well.

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, four 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. The Sentinel is non-canonical and is a development leftover.
 * 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 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. These Sentinels turn players' reticules red.
 * Normal shielded Sentinels (like the ones on the Ark, at the bridge you must activate) have relatively weak shields, as it only takes three-five 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.
 * There are two known types or 'ranks' of the regular Sentinel, one being with an energy shield, the other being without.
 * In Halo Wars, Sentinels can be produced to fight for you in the map Repository.

Related Pages

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