Precursor

The Precursors were an advanced race that preceded and were mythologized by the Forerunner. 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
Little is known about the Precursors, other than that the Forerunners believed 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 Forerunners came to view humanity shortly before the activation of the Halo Array. The Forerunners believed the Precursors had shaped the Forerunner 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
In actuality, the Precursors were responsible for seeding the galaxy with life, creating the Milky Way's diverse composition of species. Over time, they would also judge whether or not a species was worthy of the Mantle. If the species was not deemed worthy, or was found to be problematic, the Precursors would eliminate that species. Amongst the large numbers species that they had created, the humanoid species native to the world of Ghibalb, coming 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 was slated for elimination. It was at this point that the early Forerunners became aware of this threat, engaged the Precursors, and succeeded in eliminating them, 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.

Before their ultimate downfall, a plan was created and put into play by the surviving Precursors, which would involve a reunifying of the Forerunners and humanity from their shared origin. This plan centered on the targeted species to be subject to the Flood, and whether or not the humans would be able to overcome this parasitic foe, or ultimately succumb to it. While the humans would eventually prevail, their nearly concurrent defeat at the hands of the Forerunners would temporarily stifle this plan.

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 capital world of Charum Hakkor, placing it in an arena, and proceeded to communicate with the being.

Technology
Precursor technology was extremely advanced, to the point that the Forerunners labeled it Tier 0. Some Precursor artifacts were known to precede the Forerunners by hundreds of millions of years, and were durable enough to be 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.

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.

Precursor technology is believed to have been based on what Forerunners called "neural physics": The concept that inanimate matter and thought are inextricably linked. It is theorized that 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 2553 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.

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.
 * In Halo: The Fall of Reach, Dr. Catherine Halsey states the the Forerunner crystal found on Sigma Octanus IV was created by a "precursor race".

List of appearances

 * Halo 3
 * Halo 3 terminals
 * The Forerunner Saga
 * Halo: Cryptum
 * Halo: Primordium