Forerunner defense constructs controlled by Professor Anders.
Exclusive to Professor Anders, the Protector Sentinel is a flying Forerunner drone armed with an energy-based sentinel beam that is effective against all enemies. It can be upgraded to a sentinel swarm, which adds smaller Aggressor drones as an escort. Sentinels explode in an EMP blast upon death, stunning nearby enemy vehicles.
Professor Anders has shown a remarkable ability to manipulate Forerunner command codes and bypass directive locks, giving her the ability to bind the Ark's own defense constructs to her will. The full extent of her capabilities have yet to be tested, as the Professor's attention is constantly diverted to more pressing needs like finding a way home.