Monitor
From Halopedia, the Halo wiki
The Monitor is a probably a hundred thousand year old AI. The only Monitors that are known are 2401 Penitent Tangent and 343 Guilty Spark. Both Monitors control the Sentinels and Sentinel Majors. It is not known if 2401 Penitent Tangent can control the Sentinels and Sentinel Majors from the clutches of Gravemind, at the bottom of Delta Halo's Library. While 343 Guilty Spark glows blue, 2401 Penitent Tangent glows red. There's a question about 343 Guilty Spark. What was he guilty of??
