Maethrillian, commonly known as the Capital, was the political center of the Forerunner ecumene. It housed the Ecumene Council, the main governing body of the ecumene, as well as the Capital Court. It was a massive structure, greatly exceeding the scale of even the Halo installations.
Maethrillian was one of the oldest structures in the ecumene, having been constructed roughly around 125,000 BCE. Unknown to most Forerunners in later times, the Capital was constructed around the physical component of the Precursor construct known as Abaddon, discovered by early Forerunners on a distant world. A powerful intelligence closely linked with the Domain, Abaddon was believed to have taught ancient Forerunners to access the Domain, which would later become a keystone of their civilization. As the construction of the Capital progressed, Abaddon was contained within a chamber of relics known as the Mysterium and held secret from most of the ecumene apart from senior members of the Ecumene Council; however, scattered reports of its existence gave rise to the legend of “the Organon”, a Precursor artifact that controlled all others.
Despite its immense size, the Capital itself had a comparatively small population of several hundred thousand Forerunners, mostly high-ranking Builders. Trillions of ancillas served in the capital in the Capital metarchy, an enormous network coordinated by a Metarch-level ancilla that answered to the First Councilor. It was one of the most trafficked locations in the ecumene, with constant security patrols, sentinels, and transports swarming around it.Within five thousand kilometers of the structure, incoming vessels would be carefully screened by sensory fields and directed by hard light docking nets.The Capital’s system housed multiple slipspace portals that linked to many worlds, as well as the Ark.
During the Forerunner-Flood war, Maethrillian became the site of an enormous battle as Mendicant Bias, having been persuaded into rampancy by the Primordial, attempted to deliver a crippling blow to the Forerunner leadership. Even though the Forerunners responded with heavy force, Mendicant Bias was successful in firing Installation 07 in the system, sterilizing the Capital and eliminating most of the Ecumene Council. Although the Council would be reconstituted shortly afterward as the “New Council" from survivors found deep within Maethrillian, they would not return to the former capital. With the Flood having overrun the Capital system, strategic command of the ecumene was moved to the greater Ark.
Over a century after the Halos’ firing, a group of surviving Forerunners led by the IsoDidact returned to Maethrillian to restore the Domain, which had been rendered inactive by the Halo Array. While attempting to access the Mysterium, the Forerunners were confronted by a manifestation of Abaddon. Embittered and corrupted by the damage wrought upon it by the Halos, the Precursor intelligence intended to judge the Forerunners for this sins. However, the Forerunners managed to enter the Mysterium and discovered Abaddon’s physical location. Inserting a deadbolt key into the artifact, Growth-Through-Trial-of-Changesuccessfully restored the Domain, simultaneously causing Abaddon to vanish. Their mission a success, the surviving Forerunners returned to Installation 00 and left the galaxy shortly afterward.
Long after the Forerunner-Flood war, the Covenant learned about the existence of Maethrillian, revered as the fabled throne of their gods. In 2552, Supreme Commander Rho ‘Barutamee found Forerunner artifacts after an engagement with the UNSC. That led him to think he was close to finding Maethrillian. Ultimately he arrived at Reach, which he attacked with restraint, preferring to localize important Forerunner artifacts first. However, he never accomplished his secret objective, as his plan was foiled by NOBLE Team, and the rest of the Covenant did not pursue his goal.