Forerunner

The Forerunners were an ancient civilization of technologically advanced beings who disappeared from the galaxy around 100,000 years before the present. Their feats of technology, architecture and megascale engineering are still without parallel, as demonstrated by many artifacts and installations that remain throughout the known galaxy. The Forerunners' interstellar empire was known as the Ecumene and encompassed millions of worlds at its height. Though records of the Forerunners are few and at times contradictory, surviving archives indicate that they based their society and way of life around the Mantle, the belief that it was their duty to hold stewardship over all other life. The Forerunner civilization came to an end after they encountered an extragalactic alien parasite known as the Flood; after three centuries of desperate war, the Forerunners activated a galactic network of superweapons known as the Halo Array as a last resort, wiping out all life in the galaxy. What became of the surviving Forerunners, if there were any, is uncertain.

Evidence of the Forerunners' civilization and technological prowess would later be discovered by younger races, some of whom would come to conclude those artifacts could only have been forged by divine hands. Such beliefs eventually gave rise to the alien theocracy known as the Covenant, who based their religion around a highly mythologized reading of Forerunner history. The Covenant regarded all Forerunner technology to be holy, deriving much of their own technological base from Forerunner artifacts, and most notably believed the Forerunners had ascended to godhood through the firing of the Halos; by repeating the process, the Covenant believed they could follow in the Forerunners' footsteps. United under this promise, the Covenant flourished for over 3,400 years, converting multiple species to its cause before being shattered by a violent civil war.

Prior to the Human-Covenant War, humanity only had circumstantial evidence of the Forerunners; various vague artifacts and a handful of astronomical oddities hinting at ancient megaengineering. A small number of larger discoveries were cordoned off by the Office of Naval Intelligence as black sites over two centuries of interstellar exploration, though there was no consensus whether these findings were the works of one or more civilizations. The UNSC first learned of the Forerunners as a distinct culture through the second-hand knowledge of the Covenant, though it would not be until the final months of the war that the human scientific community was able to start building a more complete picture of the ancient civilization through data from various operations on Forerunner installations.