Sentinels were designed to guard the Halo Installations and many 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 energy weapons called Sentinel Beams. The Sentinels were constructed by the Forerunner for the purpose of fighting the Flood The reason behind this was that no more Forerunner would be infected by the Flood, thus slowing down their growth and consumption. When not engaged in combat, they contract into smaller forms, concealing their weaponry and using smaller lasers to repair and manage the area around them. 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. Sentinels are available as mercenary units in Halo Wars on certain maps and come in several forms.
Sentinels have been known to have been created long before Installation 04 was ever even thought of. 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 protectors of the Halo installations, and are controlled by the Monitor of the Installation. Their prima ry 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 at the Seraph, the same as the ones used by the Constructor drones in the level Sacred Icon. This shows that at least some Sentinels serve to repair functions. You can also see the Sentinels carrying gas canisters.
They are equipped with an energy weapon called a Sentinel Beam. The Sentinel Beam comes in 2 types, red and blue, the blue is more powerful than the red, though it overheats much faster. In Halo 2 and Halo 3, after a Sentinel is destroyed, these weapons can be picked up and wielded by a foot soldier. In Halo: Combat Evolved, when destroyed, there is no weapon to pick up. 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 hit will send it flying a few feet away, giving off a small EMP blast when it explodes. During the last level of Halo 3, Halo, the Sentinels fire at your Warthog and seem to push the Warthog whilst in the air. Despite it 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.
The Needler Sentinel can only be found in the level, Quarantine Zone, playing on any difficulty. There is a lone Sentinel which shoots Needler rounds on the level and, when destroyed, it drops a Needler gun. It can be found on the leftmost side of the first of the glowing orange indoor areas within the ruins of the Sentinel Constructor Factory. Note that this gun is perfectly ordinary, unlike the Scarab Gun. This Sentinel is an oversight from Bungie's tests with Sentinels using different weapons, and was meant to be deleted. This could be because that it would not be logical for Sentinels to acquire Covenant weapons. It is equipped with Energy shields, and has the same boost ability and flight capabilities that an ordinary Sentinel Major has. The only difference is the Needler rounds it fires.