http://img32.exs.cx/img32/817/penitenttangent7dz.jpg
The Monitors were placed on the Halos by The Forerunners to service and maintain the installations, and to make sure that The Flood stayed imprisoned. In case of a catastrophic outbreak, the Monitors seek out a "Reclaimer" to activate the installions and eliminate all life in the galaxy.
Known Monitors: 343 Guilty Spark 2401 Penitent Tangent