Aggressor Sentinel
From Halopedia, the Halo wiki
A Sentinel is a primordial Forerunner-constructed automated electronic construct with limited Artificial Intelligence. They were designed to guard the Halo installations and other Forerunner constructions, and are under the jurisdiction of the local Monitor.
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.
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.
The Sentinel Beam is also available in Halo 2's multiplayer. It can be found inside the bases on Backwash.
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.
There is also 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.
