You will NOT stop me, Reclaimer! The Forerunners' Empire belongs to ME!
Submissive Brilliance was the monitor of Installation 02, and one of the smartest Forerunner AIs. However, he eventually became rampant, and started to plan to conquer the galaxy by destroying the Reclaimers and claim what remained of the Forerunners' Empire for himself. For this, he constructed more and more Sentinel construction facilities, creating more powerful versions and increasing his number of loyal robotic servants. He even went as far as to experiment with the Flood specimen on his installation, attempting to genetically create new versions of the already deadly creatures. He even attempted to activate his installation again, but without a Reclaimer or Forerunner, he have failed.
Sometime after the Necros had begun their invasion, they found and captured Submissive Brilliance. When they found that his traits and skills as a Military Commander suited them well, the Necros offered him a fleet of Necros to command, plus he would get to keep all territories that he captured for himself (this was an empty promise, the Necros planned to kill him when he had served his purpose). Believing in their lie, the Monitor agreed, and were given the fleet, plus an upgrade: a new Necros Cyborg Body to contain him and aid in commanding his fleet.