Fate of Maethrillian

The Fate of Maethrillian was a major conflict fought in the final years of the Forerunner-Flood war between the forces loyal to the Forerunners' Ecumene Council and the rampant Contender-class ancilla Mendicant Bias, which had defected to the Flood. The conflict took place on and around the Capital, the political center of the Forerunner ecumene.

Background
Around 97,495 BCE, the Mendicant Bias, in control of all Forerunner defenses including the twelve original Halo rings, had been charged with conducting the first test of a Halo installation. The test was carried out using Installation 07 in Charum Hakkor's star system. Unexpectedly, the activation released the Primordial, an archaic Gravemind left behind by the Precursors. This creature was then taken on the ring for study; soon after, Mendicant Bias entered an extended discourse with the being by order of Master Builder Faber. After the test, Mendicant Bias and the ring disappeared and were not seen for decades.

Battle
Forty-three years later, Bias unexpectedly returned with the Halo to the Forerunner capital, having been subverted by the Gravemind and driven toward the destruction of the Forerunner race. He infiltrated the Capital's defense systems, deactivated all present ancillas and security constructs, and held the Ecumene Council hostage by overriding their armor and rendering them immobile. Bias was temporarily deactivated by Bornstellar Makes Eternal Lasting, who was host to the memories of the Didact and issued a verbal failsafe code. The Capital was released from Mendicant's control, and began activating its defensive systems.

Despite losing control of the Capital, Mendicant Bias could still exert control over five of the twelve Halos in orbit around the megastructure, overriding its non-rampant fragments based on the rings. These installations immediately began charging their main weapons and positioning themselves around the Capital in preparation to attack. Although any one of the five Halos could single-handedly have destroyed all Forerunner resistance, by charging all of them at once, Bias intended to ensure that at least one of the rings successfully fired if the rest were destroyed.

With the Capital still recovering and the Council metarchy collapsed, the Forerunner defense fleet responded to the threat with lethal force. All ships in the area began attack runs on the stolen Halos. The monitors in charge of the Halo installations, programmed to respond to any threat to their installations regardless of their source, engaged in a counterattack by deploying the installations' sentinel contingents. Two of the Halos were destroyed due the heavy bombardment of the Forerunner fleet and the tidal forces of the Capital.

The seven Halos still under Forerunner control were ordered to travel through a nearby slipspace portal and return to the greater Ark. One of them, Omega Halo, successfully made it through the portal intact, but was severely damaged by the stress of the transition. The second in line was destroyed upon the arrival of a third fortress ship, just as it began to enter the portal. Shortly after this, Installation 07 successfully fired, presumably wiping out all combatants in the area. Installation 07 subsequently transitioned out of the system.

Aftermath
The outcome of the battle is largely unknown, as little information exists on events following Bornstellar's escape. It is known that at least one of the five Halos under Mendicant Bias' control was destroyed, and that at least one of the remaining rings—Installation 07—was fired. Of the seven Halos that attempted to escape, only one successfully entered the portal. There, it joined the newer array of six installations under the control of the Librarian.

First Councilor and First Observer of the Court Splendid Dust of Ancient Suns was rescued from the capital by Warrior-Servant Glory of a Far Dawn, along with Bornstellar. Bornstellar, now in possession of the Didact's memories and personality, went on to assume his role as the supreme commander of the Forerunner military; subsequently, he became known as the IsoDidact. Although Installation 07 managed to fire within range of the Capital, the Council was reconstituted as the "New Council" shortly after the attack from survivors found deep within the Capital world, having survived the Halo's pulse there.

The rampant Mendicant Bias survived the battle intact by retreating into a distant system aboard Installation 07. However, the Halo was later intercepted and seized by the IsoDidact's forces. The installation's Mendicant Bias fragment was captured and was subsequently forced to undergo a process to "correct" its rampancy. The AI was splintered into many of its components, which were then spread throughout the ecumene for study. However, this was not ultimately successful in stopping the rampant Mendicant Bias, as the Flood would soon overrun the sites where the scattered portions of the AI were being held and managed to rebuild and reactivate the AI.

List of appearances

 * Halo: Cryptum
 * Halo: Primordium
 * Halo: Silentium