From Halopedia, the Halo wiki
Research on the Forerunner shield world now called Trevelyan has cultivated numerous findings for the Office of Naval Intelligence. One of the most practical has been the discovery of a native armor system utilized by Forerunner Prefects late in their civilization's history. The Watershed Division leveraged local Huragok to reverse-engineer the original designs, grafting the abandoned armor system into baseline Mjolnir standards for Spartan-IV use. It was tested at Denegroth, West Conamara, on Europa.
Prefect is one of the three armors from the Infinity Armor Pack. Like the other two armor sets, it is not usable in Spartan Ops in Halo 4. Although the Prefect was unavailable during initial release in The Master Chief Collection, it was made available in Season 7: Elite.
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