The Fate of Maethrillian was a large-scale but brief battle between the Forerunners and the Halo Array under command of Mendicant Bias several years prior to the firing of the halos during the Great Purification in 97,445 BCE. After the firing of Installation 07 during the Battle of Janjur Qom against the San’shyuum, the Master Builder was removed from power and put on trial on Maethrillian for crimes against the Mantle. In addition, those in charge of the Forerunner government brought all eleven remaining Senescent Halos to the system in order to discuss decommissioning them.
Not long after the trial began, Mendicant Bias and Installation 07 arrived in the system and tried to take control of the entire array. A fierce battle began, with the Forerunner military trying to destroy all halos, the installation monitors trying to take control and transport the rings out of the system, and Mendicant Bias trying to fire the halos on the capital. Though Mendicant Bias was not able to take control of many of the halos, he was able to fire Installation 07 on Maethrillian, wiping out all Forerunners on the world and in the system, leaving the Forerunner ecumene in ruins. Though most Forerunners in the system were killed, a handful managed to escape before Zeta Halo fired, including the IsoDidact.