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 partly misinterpreted and 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.

History
Though little is known of the Forerunners themselves, a fair number of individuals have been identified by name or title in surviving records. The Kandonom Codex, for instance, names the Didact, the Librarian and the Master Builder as individuals of note in the Forerunners' final days. Numerous other names are also recorded; these would be ended up codified in the Covenant pantheon (specifically the pluralist doctrines, which emphasized the individual personages of gods) as greater and lesser Forerunner deities.

Since the Forerunners' fall, various younger civilizations have come across the relics they left behind. While the Forerunners once controlled millions of systems, many of them highly built-up with artificial worlds, stations and more, a scant few of them survive to the modern age. It is known with relative certainty that in their war with the Flood, the Forerunners used scorched-earth tactics to deny the Flood access to their technology and even entire populations. Systems doomed to become fodder for the parasite would be systematically sterilized and entire artificial worlds dismantled by nano-viruses introduced into their self-repair systems. Another factor to explain the relative scarcity of intact Forerunner technology in the present is simple decay: while advanced, even Forerunner technology decays over tens of millennia without a self-replenishing supply of sentinels to repair it, which is the case on the vast majority of sites.

By the time of the rise of the Covenant, other civilizations active in the Orion Arm prior to them — the Jehioi, the Optem, and potentially others — had already stripped many relics clean of useful technologies. Consequently, intact and functioning Forerunner technology is exceedingly rare, especially on a large scale. Particularly in the regions near the Orion Complex, there are worlds with entire Forerunner megalopolises that now lay cold and dead, free of anything but a scant few artifacts. Most examples of Forerunner technology encountered by the Covenant are mundane items found in ruins of what are usually civilian sites and drifting starships or long-abandoned stations; this has not stopped the Covenant from making use of such technologies, however. Intact military technology is rare, but is found at times; the Covenant did discover less than a handful of shield worlds in their history, though only a pair of them were fully and consistently accessible. Another risk on installations of any importance are native defenses, which are at times so powerful as to stymie any attempts at incursion.

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. According to Covenant doctrine, such transformations demonstrated the Forerunners' divine mastery over mind, body and soul: they could sculpt flesh as a sculptor does stone, each individual being shaped to perfection according to their station.

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, as do the head shapes; this is attributed to either creative interpretation, extreme ethnic variety, or changing fashions in biological modification throughout different eras.