Phoenix Logs/Forerunner/Controller Sentinel
From Halopedia, the Halo wiki
Forerunner system offensive/defensive drones
The Controller Sentinels’ primary function is to guard valuable Forerunner technologies on the Ark. Developed and deployed to counter the escalating threat from the UNSC and the Banished, these flying Sentinels are loosely 'tethered' within set radii of important sites and will attack anyone that comes close, or attempts to take control of the sector.
An impenetrable energy shield protects the front of each Controller Sentinel, an improvement on the already powerful shields used by the Enforcer Sentinels whose function was to suppress Flood outbreaks. Underneath it fires a continuous Sentinel Beam to prevent enemies from hiding in its blind spot, but its primary weapon is a devastating supercharged Sentinel Pulse.