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.

Biology and appearance
Due to the extreme scarcity of intact records and data, the exact anatomy of the Forerunners remains a mystery. Most Covenant sources agree that in their most common biological forms, the Forerunners were upright, four-limbed bipeds with a humanoid body plan, though it is commonly agreed that they could alter their forms at will, either subtly or extensively, to suit different roles, preferences and habitats. It is known that the Forerunners wore personal body-assist armor for much of their lives, with the armor having enormous cultural and social significance to the extent that no representation of an unarmored Forerunner survives. Likewise, no detailed depiction of a Forerunner face has ever been uncovered, as the few images of Forerunners depict them with what appear to be visors or energy fields covering their faces; it has been theorized that the Forerunners may have had a cultural taboo on showing their faces in public, or simply did not regard them as important. A few abstract or artistic representations seem to suggest the presence of two to four eyes, two nostrils, and a mouth. However, the exact arrangement and proportionality of these varies greatly by source, which is attributed to either creative interpretation, extreme ethnic variety, or changing fashions in biological modification throughout different eras.