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.

Known history
Little is known about the Precursors, other than that the Forerunners believed that their 'Mantle' (their role of guardianship of the galaxy and of all life) was handed down to them from 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 viewed humanity 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. Prehistoric humans also claimed to be the sole inheritors of the Mantle, something the Forerunners considered heretical.

The majority of this information is mythology, some perhaps with a historical basis and most invented to explain unknown information. According to the Timeless One, the last remaining Precursor as of the Forerunner-Flood war, the relation between the Forerunners and Precursors was far removed from the ideas held by Forerunners. Rather than simply disappear or "transcend" after passing on their Mantle, the Precursors created the Forerunners millions of years before, who rose up and ruthlessly destroyed their creators, facts that are at odds with the supposed pacifism of the Mantle.

Physiology
All that is known about the Precursors' appearance is based upon the prisoner at Charum Hakkor, the last known Precursor. Precursors were massive in size; the mold encapsulating the Timeless One outlined a being approximately fifteen meters tall and eleven meters wide. They were vaguely humanoid, with four arms, two legs and an insectoid head. A long, segmented "tail" was attached to the base of the skull, tipped with a meter-long barb. Each hand had three fingers and a central opposable thumb.

The face of a Precursor has been likened to that of a eurypterid, or "sea scorpion." Their heads were flattened, with oval compound eyes and (presumably) mandibles or similar mouth parts.

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 sometimes cycled inside planets' crusts by plate tectonics. Precursor artifacts were often encountered by the 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 Precursors structures were the enormous orbital arches and unbending filaments, which were used to connect entire worlds. 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 panels if broken.

Precursor technology is believed to be 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 the one few ways to destroy or otherwise damage Precursor artifacts. This could also explain the lack of any Precursor ruins in modern society (circa 2552 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

 * Bestarium
 * The Forerunner Saga
 * Halo: Cryptum