Sentinels
From Halopedia, the Halo wiki
Summary
The Sentinels are the guardians of the Halo installations, and are 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 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.
