Precursor

"We are the last of those who gave you breath and form, millions of years ago. We are the last of those your kind defied and ruthlessly destroyed. We are the last Precursors. And now we are legion."

- A Gravemind to the Ur-Didact.

The Precursors were an advanced race that preceded and were mythologized by the Forerunners. The Forerunners believed them to be theoretical "Transsentient" beings, having the ability to travel among galaxies and accelerate the evolution of intelligent life. While the Precursors themselves had long since disappeared during the time of the Forerunners, evidence of their technological and architectural achievements remained on worlds they had once colonized.

As understood by the Forerunners
The Forerunners knew few concrete details about the Precursors, and most of their supposed knowledge came from the mythology they had constructed around their creators. Their most central belief was that the "Mantle" (the role of guardianship of the galaxy and of all life) was bestowed upon them by the Precursors. From a Forerunner perspective, the passing of the mantle secured the Precursors' legacy in the form of the Forerunners, continuing their work after they departed, much in the same way humanity was appointed to be the Forerunners' inheritors shortly before the activation of the Halo Array. The Forerunners believed the Precursors had shaped the Forerunners in their own image, and some even suggested that they may have done the same with humans. By the time of the Human-Forerunner war, prehistoric humans also claimed to be the sole inheritors of the Mantle, something the Forerunners considered heretical at the time. The majority of this information was mythology with an unspecified amount of historical information, serving the purpose of providing a set of explanations for the innumerable mysteries surrounding the Precursors. The facts, as relayed to the Didact by the Primordial, supposedly the last remaining Precursor as of the Forerunner-Flood war, were very different from what the Forerunners had ultimately chosen to believe.

The Truth
"We are the Flood. There is no difference. Until all space and time are rolled up and life is crushed in the folds... no end to grief, war or pain. In a hundred and one thousand centuries... unity again, and wisdom. Until then - sweetness."

- The Primordial speaking to the Didact.

The Precursors were an incredibly advanced race of beings who explored many galaxies and seeded them with life over the course of many billions of years. As true transsentient beings, they were not tied to any particular physical form, assuming any shape as they saw fit; they would allow themselves to die away and be evolved anew over and over again, taking on numerous incarnations both physical and pure thought. They lived through different stages of technological and cultural development numerous times, being at times hyper-advanced and spacefaring and at others living primitively and remaining confined to their worlds.

The Precursors stored all of their collated knowledge - amounting to a hundred billion years, with most of the information gathered originating from the beginning of the universe, or earlier - in a vast reserve contained in Precursor constructs in the Milky Way galaxy. The field projected by this store of knowledge was used by the Forerunners in form of the Domain, although both its origin and true nature were unknown to them.

The Precursors were responsible for seeding the Milky Way with life, creating the galaxy's diverse composition of species. Over time, they would also judge whether or not a species was worthy of the Mantle, their assumed role of guardianship of all life. Amongst the large numbers of species that they had created the humanoid species native to the world of Ghibalb, who would come to be known as the Forerunners, were next chosen for this task. This species was eventually judged to be unworthy of taking on the Mantle. The Precursors announced their decision to the Forerunners, at which point they furiously retaliated against the Precursors, and succeeded in eliminating their creators, first in the Milky Way and eventually in the satellite galaxy of Path Kethona, though a small but unspecified number managed to escape the Forerunner's campaign of extermination. Prior to their destruction, however, the Precursors had created and shaped yet another potential holder of the Mantle, a collection of humanoid species referred to as humanity, hailing from the planet known as Erde-Tyrene.

A few Precursors either escaped or were spared by the Forerunners. They either went into suspended animation or became dust that would regenerate into their past forms. However, over time the dust became defective, failing to reconstitute the Precursors and instead inducing madness and mutations in life-forms that came in contact with it. Far from accepting failure and extinction, however, the Precursors viewed this as a means to bring unity to the galaxy, as well as punish the Forerunners for their insolence. With their new form - the nascent stage of the Flood - the Precursors vowed that none of their creations would rise against them again.

In 110,000 BCE, an exploratory group led by Yprin Yprikushma discovered a small planetoid at the edge of the Milky Way, and proceeded to study and excavate it. Hidden within it was a large stasis capsule containing an ancient being, who they interpreted as being a Precursor. They transported the capsule and its captive to the human capital world of Charum Hakkor, placed it in an arena, and proceeded to communicate with the being.

When the Flood first appeared in the galaxy, several humans questioned the imprisoned being, whom they named the "Primordial," as to the nature of the parasite. The being's response horrified the humans so deeply that many of them committed suicide. Around ten thousand years later, the IsoDidact interrogated the Primordial on Installation 07, the being claimed the Precursors had fled beyond Forerunner reach, and that they continued their creation and manipulation of life in the galaxy. The Primordial also implied that the Precursors and Flood were synonymous, refusing to elaborate any further.

Technology
Precursor technology was extremely advanced, to the point that the Forerunners labeled it Tier 0. Precursors were believed to have based their technology upon what Forerunners called "neural physics": The concept that inanimate matter and thought are inextricably linked and that universe itself was effectively a living entity. The Halo Array, being neurological weapons, are one of the only ways to destroy or otherwise damage Precursor artifacts. This could also explain the lack of any Precursor ruins in modern society (circa 2558 CE) as most, if not all, would have been destroyed when the Halo Array fired and sterilized the galaxy at the end of the Forerunner-Flood war.

Some Precursor artifacts were known to precede the Forerunners by hundreds of millions of years, and were durable enough to survive being cycled inside planets' crusts by plate tectonics. Precursor artifacts were often encountered by Miners, who obtained and recorded them but rarely held any particular interest in them. The most coveted Precursor artifact among the Forerunners was known as the Organon, which was allegedly capable of activating all other Precursor artifacts. In truth, the Organon was synonymous with the Domain.

Among the most impressive of the Precursors' structures were the enormous orbital arches and unbending filaments, which were used to connect entire worlds and solar systems. Precursor technology was amazingly durable, and was almost totally impervious to all forms of conventional damage. However, the Halo Array's energy pulse has been shown to be able to utterly destroy Precursor structures. When Charum Hakkor was used as a test bed for one of the Halos, the weapon's firing shattered every Precursor artifact on the planet. Precursor construction material was gray-silver, and would shatter along crystalline planes if broken.

Toward the end of the Forerunner-Flood war, the Flood began to employ formerly inert Precursor artifacts against the Forerunners, being able to control them due to its Precursor origins. These artifacts - mainly star roads - were what eventually turned the war in the Flood's favor as they were capable of effortlessly either outright decimating Forerunner fleets or remotely deactivating their weapons, AIs or shields, rendering them helpless against the Flood.

Trivia

 * The term precursor means "one that precedes and indicates the approach of another". This is also the definition of the term forerunner.
 * The Latin root word "trans" means "across" or "beyond." "Sentience" is the ability to experience sensation, and is often used to imply sapience, the ability to think. A transsentient being may thus be considered beyond any recognized being and be on the level of godhood.

List of appearances

 * Halo 3
 * Halo 3 terminals
 * Halo: Cryptum
 * Halo: Primordium
 * Halo: Silentium