From Halopedia, the Halo wiki
The myrmidons were Forerunner special forces who guarded members of the Ecumene Council, as well as vital installations. Their rate and dispensation was never definitely established within the ecumene.
Splendid-Dust-of-Ancient-Suns, the First Councilor of the Ecumene Council, held a contingent of myrmidons, which he would later deploy to guard the Halo Array following the incident on Janjur Qom.
In Greek mythology the Myrmidons were a legendary people native to the region of Thessaly.