The Forerunners (Latin Primoris prognatus, meaning "Firstborn") were an ancient species of extremely technologically advanced beings whose empire—known as the ecumene—encompassed three million fertile worlds in the Milky Way Galaxy. The Forerunners reigned for millennia as the ascendant civilization in the galaxy until the activation of the Halo Array, their Pyrrhic solution to halt the Flood, in 97,445 BCE. Although they themselves are essentially extinct, evidence of their existence remained, spread across the galaxy in the form of numerous creations, installations and artifacts.
The Forerunners took upon themselves the role of caretakers of the galaxy and believed that this "Mantle" had been passed down to them by the Precursors, an even more powerful race that preceded the Forerunner civilization. "Forerunner" is a literal translation of the species' name for themselves; they identified themselves as such because they believed that they held an impermanent place in the universe's Living Time, though, in time, they too would be succeeded by other, superior races. Before their disappearance from the galaxy, the Forerunners would name humanity their successors, identifying them with the title "Reclaimer".
In modern history, the alien hegemony known as the Covenant mythologized the Forerunners and revered them as gods, deriving much of their own technology from Forerunner artifacts found throughout the galaxy, and held that any who defaced these relics were heretics.
“One hundred thousand years ago, the galaxy was populated by a great variety of beings. But one species—eons beyond all the others in both technology and knowledge—achieved dominance. They ruled in peace but met opposition with quick and brutal effectiveness. They were the Forerunners—the keepers of the Mantle, the next stage of life in the Universe's Living Time. And then they vanished.”
The Forerunners were created by the Precursors, a powerful race responsible for seeding the Milky Way Galaxy with life and creating many other species, fifteen million years ago. The Forerunners were seeded on the planet Ghibalb in the Orion complex from a base stock that also produced humanity. The world served as the hub of the Forerunners' fledgling interstellar civilization, comprised of twelve systems within the nebular complex, until it was rendered uninhabitable by an astroengineering disaster. The Forerunners had attempted to manipulate several stars in their region of space but accidentally caused a series of supernovae that resulted in their planet being burned to a cinder by radiation. This event nearly caused the Forerunners' extinction.
For ages, the Forerunners and humans evolved alongside each other with the Forerunners eventually overtaking their peers in wisdom and technological development. The Forerunners would master many arts and sciences at the feet of the Precursors, serving as their assistants and adjutants while the strange workings of their creators' minds and artifacts eluded even the Forerunners' keen intellects. As their interstellar civilization developed, the Forerunners came to believe they would be judged worthy to be the inheritors of the Precursors' Mantle of guardianship of all life. However, according to some records, over ten million years ago, the Forerunners achieved interstellar travel on their own and found remnants of their creators' existence long after the latter had left the galaxy to observe it from Path Kethona. During this time, the Forerunners discovered and claimed the Mantle as their own. The Precursors did not approve of this and made the decision to strip the Forerunners of the Mantle, wipe out the Forerunners, and pass down the Mantle to humans instead. Other records state that the Precursors originally chose humans to bear the Mantle without the Forerunners ever holding it beforehand. Provoked either by this revelation alone, or the Precursors' alleged decision to eradicate the Forerunners as a "problematic" species, the Forerunners responded by overthrowing their creators, exterminating most of them in retaliation.
The Forerunners, led by the then-dominant rate of Warriors, began their thorough campaign of extermination in the Milky Way galaxy and later chased the last Precursors down to the satellite galaxy of Path Kethona. There, they would finish their work, eradicating all but a few Precursors who managed to elude them. However, some Forerunners began to question the ethics of the genocide and refused to take part; many were summarily executed, while others were exiled on a barren planet in Path Kethona without any advanced technology. This population would survive for millions of years, preserving the knowledge of their origins in a biological reservoir of ancestral memory. However, they would remain as the last survivors of the fleet that traveled to Path Kethona, as those who had carried out the genocide until the end never returned to the Milky Way.
Records of this war were gradually lost to time over millions of years, and Forerunner civilization as of the Forerunner-Flood war believed that the Precursors had simply disappeared after fulfilling their ultimate goal: creating the Forerunners to be their successors as holders of the Mantle.[15] For several millions of years after the annihilation of the Precursors, the truth was purposefully suppressed by those in power; there were some who attempted to uncover the true relationship between the Forerunners and the Precursors, but they were discreetly silenced, mainly by Warriors and Builders.
Over the course of their history, the Forerunners established empires, fought wars, built worlds, and fought each other in a number of civil wars, one of which took place over half a million years before their war against the Flood. Their own various civilizations grew, splintered, merged and evolved until all eventually converged into the ecumene. To safeguard the Orion complex—the center of the ecumene's governance—during these conflicts, the Forerunners constructed the Maginot Line, an immense array of defensive installations spread out in a sphere which completely surrounded the Orion complex. This defense was meant to repel any possible enemy incursion, but it eventually proved vulnerable against an unconventional attack. During the civil wars, the Builder rate first rose to supremacy and initiated a cultural purge of the other rates, such as Warriors and Miners, which involved the suppression of their ancient rituals.
The Forerunner civilization had cemented its place as the preeminent civilization in the galaxy by roughly 150,000 BCE, around the same time humanity first began moving its latest interstellar civilization away from their homeworld, supposedly to escape early Forerunner control. The Capital, the center of the Forerunner ecumene and the seat of the Ecumene Council, was constructed around 122,990 BCE.
After their disappearance, the Forerunners left behind a great deal of artifacts, many of the more significant ones overseen by their dedicated monitors which would react to outside visitors in accordance with their programmed instructions. The mystery surrounding the Forerunners and their technology would later become a source of worship, legend, advanced technology, or combination thereof for the various future civilizations which eventually stumbled upon some of the many Forerunner artifacts across the galaxy.
The first known species to make use of reverse-engineered Forerunner technology were the Sangheili and San'Shyuum, having evolved on worlds rich in Forerunner artifacts. Regarding these artifacts and their creators with religious reverence, many wars broke out over whether the relics should be left in peace or used to advance their own technology. Those who believed the latter came together to form the hegemony known as the Covenant, who together created a complex religion based around venerating the Forerunners as gods. Due to misinterpreting the ancients' records, the Covenant believed that the Forerunners disappeared from the galaxy after the Halo Array elevated their species to a state of trans-sentient godhood. As such, they appropriated many Forerunner technologies and artifacts, with the use of Luminaries that allowed them to track Forerunner-made objects all across the galaxy. The most holy of these artifacts, the "Sacred Rings" or Halo installations, were constantly searched for 1700 years, since the Covenant believed themselves to be the chosen inheritors of the Forerunners' legacy, and by locating and activating the Halo Array, they believed that they too could follow the Forerunners into godhood. This was referred to as the "Great Journey" by the Covenant races. This mimicry of the Forerunners' remnants caused the Covenant to rise to a high technological tier very quickly but additionally stunted their innovation of said technology and kept them from uncovering its true potentials.
As for humanity, they had been chosen by the Forerunners to carry the title of "Reclaimer" and eventually take over the Mantle. The Librarian had imparted humanity with a species-wide geas; these genetic instructions include an innate familiarity with Forerunner technology. However, humans themselves were unaware of this and came into contact with Forerunner artifacts relatively late in their history, after they had colonized many planets. In 2491, a collective of human artificial intelligences uncovered the presence of Forerunner relics on Onyx and kept them hidden in the meantime for fear that humanity was not yet ready to use such advanced technology. As such, archaeological excavations did not uncover them until 2511, and the United Nations Space Command's Office of Naval Intelligence responded by hiding the planet's location and securing the area. However, little progress was made in translating any of the relics. Other relics were discovered on Reach, Earth, and many other colonies.