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 achieved a higher level technological advancement than the Forerunners, being Tier 0, or Transsentience on the Forerunners' Technological Achievement Scale.

History
Even less is known about the Precursors than about the Forerunners, but information that can be found by text from the logs stored in the Terminals around the Ark indicates it is possible that the Forerunner 'Mantle' (Guardians of the Galaxy and all life) was handed down from "legends of the past": the Precursors. It has been presumed that the Precursors viewed the Forerunners as their own legacy, 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.

However, as the last Precursor was encountered by Didact in a sealed cage on Charum Hakkor (the last human stronghold in the Human-Forerunner War), it stated that the Forerunners were indeed created by the Precursors millions of years before and in turn, the Forerunners rose up and ruthlessly destroyed them.

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, approximately fifteen meters tall. 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 mouthparts.

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.

Some Precursor technology is believed to be based on what Forerunners call "Neural Physics": The concept that inanimate matter and thought are inextricably linked. It is theorized that the Halo Array, being neurological weapons, are the only way to destroy or otherwise damage Precursor artifacts. This could also explain the lack of any Precursor ruins in modern society (2552 CE) as they would have all 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." The term supports that it surpassed the Forerunner.
 * The Precursors were the only known civilization to achieve Tier 0 of the Forerunner Technological Achievement Tiers and 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