Sentinels
From Halopedia, the Halo wiki
Summary
The Sentinels are the guardians of the Halo installations, and can be controlled by the Monitor of the Installation.
They are hovering robots equiped with an energy weapon called a Sentinel Beam. These beams are used to control The Flood, however they can only manage them for short periods. In Halo 2, after a Sentinel is destroyed, these beams can be picked up and wielded by a foot soldier.
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 Sentinels fly, they make a sound similar to police sirens, presumably because are the "police" of the Halos.
The sentinel beam is also available in Halo 2's multiplayer. It can be found inside the bases on Backwash.
