POPULATION: Unknown, likely 0
Maethrillian was the home of the Forerunner Ecumene, the center of governance during the time of Forerunner dominance, prior to their war with the Flood and the firing of the Halo array. The "planet" was actually made of several circular plates attached along a central spine, which gave the appearance of a segmented and separated planet. Some time prior to the firing of the Neoteric Halo Array, Maethrillian was attacked by the Forerunner ancilla Mendicant Bias using the Senescent Halo Array, causing widespread destruction to the planet and killing untold numbers of Forerunners, including many in top leadership positions. The planet was visited soon after the firing of the Neoteric Halo Array, and was discovered to be heavily damaged, but in the process of trying to repair itself.