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The Metarchy was the governing ancilla network of the Forerunner capital world Maethrillian, managing its various domestic and defense systems, including the Capital's sizable defense fleet. A vast interconnected network consisting of trillions of ancillas, the Metarchy was overseen by a Metarch-class ancilla, one of only five active at the time.
The Metarchy was active during the trial of the Master Builder, but was subverted early in the proceedings when 05-032 Mendicant Bias arrived at Maethrillian with a stolen Halo ring intent on wiping out the Forerunner leadership. Disabling the governing Metarch-class ancilla, Mendicant began systematically suppressing all other ancillas present on the Capital, and locking up the armor of many Forerunners on the planet.
According to Splendid Dust of Ancient Suns the Metarchy would have been destroyed when the Halo arrays in proximity to Maethrillian fire, losing millenia of Forerunner knowledge.