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 c. 100,000 BCE.

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 scattered across not only the galaxy but also outside it, namely in the Greater Magellanic Cloud.

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. According to Forerunner belief, the Precursors had peacefully passed away after they fulfilling their mission in creating the Forerunners. 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 a dubious historical basis, serving the purpose of providing a set of explanations for the innumerable mysteries surrounding the Precursors. The facts 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 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, and the Precursors instead decided that the responsibility would fall on the shoulders of another one of their creations; a collection of humanoid species referred to as humanity, hailing from the planet known as Erde-Tyrene. When the Precursors announced their decision to the Forerunners, around 10,000,000 BCE, the latter furiously retaliated against their creators and drove them to near-extinction, first in the Milky Way and eventually in the satellite galaxy of Path Kethona, though a small number managed to escape the Forerunners' campaign of extermination. According to the Precursors themselves, the Forerunners struck against their creators unprovoked, while certain Forerunners who learned the truth later on insisted that the Precursors had planned to wipe out the Forerunners first.

A few Precursors either escaped or were spared by the Forerunners. They either went into suspended animation or became dust that was meant to eventually regenerate into their past forms. However, over millions of years, 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.

Incidentally, the first beings to come into contact with the nascent Flood were early humans, around 110,000 BCE - ten million years after the Forerunners' genocide of the Precursors. The Flood initially ravaged humanity's colonies, until receding from the galaxy on its own accord, awaiting for a better moment to exact their final vengeance on the Forerunners. Around the same time, a human 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 whom they later named the Primordial. They transported the capsule and its captive to the human capital world of Charum Hakkor and proceeded to communicate with the being, which claimed to be the last Precursor.

When the Flood first appeared in the galaxy, several humans questioned the imprisoned being as to the nature of the parasite. The Primordial'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, receiving only vague responses as to the fate of the Precursors and their relationship with the Flood. Enraged, the IsoDidact then executed the being. The Ur-Didact later discovered that the Gravemind retained the thoughts and memories of the Primordial, demonstrating that the Flood's collective consciousness was a direct continuation of the Precursors.

Technology
"The Flood changes everything. Not just flesh. Space itself is infected. That's the power the Precursors once had... isn't it? They shaped and moved galaxies! They created us! How did we ever manage to defeat them?"

- The Ur-Didact to the IsoDidact.

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. 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. This also explains the lack of any Precursor ruins in modern society 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. The Precursors also left behind a small number of mysterious deep-space artifacts known as anchors; anomalous, large masses which released no radiation. They were generally regarded as unstable and dangerous; the Forerunners had recorded strange phenomena near the masses, including ships vanishing and surviving crews suffering severe mental trauma, requiring extensive proto-geometric therapy to restore their minds' neural topology to its normal state. Precursor construction material was gray-silver, and would shatter along crystalline planes if broken.

Among the most impressive of the Precursors' structures were the enormous orbital arches and star roads, unbending filaments which were used to connect entire worlds and solar systems. These structures were once plentiful in the galaxy, forming graceful curves across many star systems. They appeared to be mostly dormant and unchanging, apart from automatic adjustments based on changes in planetary orbits. Other types of Precursor construct included structures referred to as planetary fortresses and citadels. Toward the end of the Forerunner-Flood war, the Flood began to reactivate and employ these 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 physically tearing apart Forerunner fleets or remotely deactivating their weapons, AIs or shields, rendering them helpless against the Flood.

The Precursors' rekindled might extended beyond the mastery of technological artifacts. When moving across the galaxy, Flood-controlled Precursor constructs would convert space-time itself to be unsuitable for Forerunner slipspace drives, due to the differences between the slipspace travel used by the Forerunners and the Precursors' neural physics-based transit. This affected the galactic-scale magnetic fields that marked the borders of the ecumene's themas. The effects of the Precursors' manipulation of space could even be felt by individual beings; toward the end of the Forerunner-Flood war, Forerunners began to experience light as possessing a hostile, repelling quality.

Perhaps the Precursors' most extraordinary creation was what the Forerunners knew as the Domain; a vast reserve containing the totality of the Precursors' accumulated knowledge and experience - amounting to a hundred billion years, with most of the information gathered originating from the beginning of the universe, or earlier. This reserve of knowledge was contained in Precursor constructs in the Milky Way galaxy and projected a massive field through which the records could be accessed anywhere regardless of locality. The Domain itself was a conscious entity with motivations aligned with the Precursors themselves, although this only became apparent near the end of the Forerunner-Flood war. The Domain would later be discovered and appropriated by the Forerunners as a store of their own records and knowledge, becoming a principal element of their culture. Despite this, the Domain remained highly esoteric to the Forerunners; not even its origin and true nature were truly known. With the activation of the Halo Array and the galaxy-wide unraveling of Precursor neural physics, the Domain and all knowledge contained therein was lost along with all Precursor architecture.

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