Precursor

The Precursors were a race preceding and mythologized by the Forerunner, being theoretical "Transsentient" beings, having the ability to travel among galaxies and accelerate the evolution of intelligent life. The Precursors reached a level of technological sophistication that dwarfed even the Forerunners, capable of creating nearly invulnerable artifacts that could withstand the stresses of a planet's mantle or even connect two planets. The Forerunners considered the Precursors to be Tier 0 on their Technological Achievement scale, transsentient beings similar to gods, able to travel across entire galaxies and responsible for accelerating the evolution of sentient life. These are largely the result of the Forerunners' own mythology, however, and while the Precursors were certainly advanced the fact that so little survived their disappearance means that it is difficult to gauge how advanced they really were.

History
Even less is known about the Precursors than about the Forerunners, but information from the logs stored in the Terminals around the Ark indicate that the Forerunners believed that the 'Mantle' (guardianship of the galaxy and all life) was handed down from "legends of the past": 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 rings. 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. Most of this is mythology, some perhaps with a historical basis and most invented to explain unknown information. According to the Prisoner of Charum Hakkor, rather than disappear 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 Prisoner of Charum Hakkor 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
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 believed to be capable of activating all other Precursor artifacts.

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 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" - interestingly, this is also the definition of the term Forerunner.
 * The Precursors were the only known civilization to exceed the Forerunner in technological advancement.
 * 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. Halsey uses the term Precursor to describe the alien race that created the crystal found on Sigma Octanus IV.

List of appearances

 * Bestarium
 * The Forerunner Saga
 * Halo: Cryptum