The Forerunners 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.
The Forerunners were a bipedal species resembling humans in overall shape, though they were larger in stature. Over the course of their lives, Forerunners underwent a number of artificially-induced mutations, modifying their bodies to better suit the field of work of their rate. More specific aspects of their appearance varied with lineage, age and even personal tastes. They relied heavily on personal body-assist armor which, in addition to protecting the wearer from harm, augmented their physiology and mental faculties. Among many other advantages, it made their lifespans virtually indefinite by providing constant life support and medical care.
Before their first mutation, as Manipulars, Forerunners were tall and slender, and possessed patches of fine, purple-pink or white fur on the top of their heads, along their nape and shoulder area, as well as the backs of their hands. Their facial features were similar to those of humanity, and they were capable of making analogous expressions, but this changed as they mutated to their higher forms, which characteristically had stiffer facial muscles and a more limited ability to convey emotions. There was also a cultural stigma associated with expressing emotions; while even certain higher forms were capable of producing some semblance of a smile or laughter, for example, such uncontrolled display of emotions was regarded as barbaric. Unlike humans, Forerunners lacked pronounced noses or ears, having two slitted nostrils and a set of bulbous protrusions instead. Their skulls otherwise bore a very strong resemblance to those of humans, with subtle differences. The Forerunners also had a genetic code that was remarkably similar to humans, though they were not known to be genetically related. However the defunct Theoretical rate of the Forerunners own studies of human and Forerunner origins concluded that humanity share a genetic structure homogeneous to the Forerunners.
The Forerunners exhibited sexual dimorphism largely mirroring that of humans, with the females being more slender and somewhat smaller in stature than the males. The height of higher-form males ranged from around three to four meters; for example, the Ur-Didact was three and a half meters tall, while Wise-Hands-to-Forge, a high-ranking Builder, was around four meters. His wife, a female Builder of the same social standing, was just over two meters in height, while the Librarian, head of the Lifeworker rate, was nearly three meters tall. Males of higher forms also bore less resemblance to humans, while the females appear to have been more human-like in appearance. A large portion of this dimorphism was artificially induced with their mutations and determined by rate.
Their skin color ranged greatly, including gray, black, blue, pink, or a mixture thereof. The number of fingers Forerunners possessed varied according to their rate and form; the most common number of fingers per hand appears to have been six, with two opposable thumbs, although some Lifeworkers could have as many as seven fingers per hand. Meanwhile, the Librarian had only five fingers in each hand, although this was a deviation from the norm.
Thanks to their advanced life-support and medical technology, Forerunners could maintain a youthful appearance for thousands of years should they so choose. Even though Forerunner armor was capable of healing most injuries which would otherwise be certainly lethal, including fatal doses of radiation, on rare occasions Forerunner individuals may choose to decline basic treatment; the ancient Warrior-Servant known as Bitterness-of-the-Vanquished, for example, chose to remain blind and instead relied on her armor to provide sensory input for her.
While the Forerunners had a biological need to sleep, this necessity was eliminated by the use of personal armor which allowed them a constant continuity of consciousness without having to sleep; only on rare cases, they would experience diagnostic waking dreams.[ As such, Forerunners slept very rarely, mostly doing so by choice in instances such as long stretches of space travel, which themselves were rare due to their exceedingly fast slipspace technology.
Forerunners possessed highly advanced mental capabilities, far beyond those of a human in the case of their higher forms. These abilities were further augmented by their use of personal armor, which integrated with the wearer neurally and stored sensory information and memories, even consistently backing up the wearer's consciousness. The armor also allowed Forerunners direct information transfer to one another, which could also be employed in the form of "silent conversation". Higher-form Forerunners had the ability to process enormous amounts of information simultaneously; for example, Warrior-Servant commanders could simultaneously process the sensory input of thousands of their subordinates while coordinating large-scale battles and themselves experiencing reality in multiple separate frames of reference with variable streams of time. The use of an ancilla enabled Forerunners to recall any past event with exact accuracy and even without armor or a personal ancilla they could remember enormous amounts of precise information; for example, the Didact was able to recite a verbal control code consisting of hundreds of completely random words and numbers after thousands of years.
The Forerunners' technological achievements were almost without parallel in the known galaxy with them being rivaled by ancient humanity and the San'Shyuum before the latter two species defeat in the human-Forerunner wars and with only the Precursors being known to surpass the Foreunners technologically. They managed to create solidified surfaces out of light by using technology known as hard light, use slipspace to teleport between locations almost instantaneously, and create numerous forms of advanced machinery. Another of their greatest achievements is their ability to initiate premature stellar collapses of stars and create planets in less than 10,000 years. The Halo Array and the shield worlds are, above all others, the most significant pieces of surviving Forerunner technology. The planet Onyx and its inner Dyson sphere, Shield World 006, in particular demonstrated both their ability for engineering on a grand scale, and their near-transcendent grasp of slipspace technology. Forerunner technology is remarkably resilient, being able to remain intact for over a hundred millennia. The finding of Forerunner technology can lead to the advancement of technology by "leaps and bounds" leading to their technology being highly sought after.