- “We meet again, young one. I am the last of those that gave you breath and shape and form, millions of years ago. I am the last of those your kind rose up against and ruthlessly destroyed. I am the last Precursor. And our answer is at hand.”
The Precursors were a race of beings preceding and possibly mythologized by the Forerunners, who believed them to be theoretical "Transsentient" beings, having the ability to travel among galaxies and accelerate the evolution of intelligent life. The Precursors achieved a higher level of technological advancement than the Forerunners, being Tier-0, or Transsentience on the Forerunners' Technological Achievement Tiers.
The logs stored in the Terminals around the Ark record that the Forerunner 'Mantle' (Guardians of the Galaxy and all life) may have been handed down from "legends of the past," the Precursors. The Precursors presumably viewed the Forerunners as their own legacy. They expected them to continue their work after they departed—the same way that Forerunner lifeworkers (including the Librarian, or the Lifeshaper) viewed Humanity before the activation of the Array.
The Primordial, a Gravemind that identified itself as the last Precursor, revealed that the Precursors had been ruthlessly killed in a war with the Forerunners millions of years before the events of Halo and that "their answer is at hand" - implying that a proto-Flood form (described as a fine dust contained in hundreds of glass cylinders) was created as revenge or as a failsafe. The fact the Gravemind considered itself a Precursor implies that the Precursors created it for revenge against the Forerunners. If so, then they were entirely successful.
However, in Halo: Silentium the Flood is revealed to be another form of Precursor made via the powder referenced to in Halo: Cryptum that was also revealed to be actual Precursor desiccated remains. They were annihilated by Ancient Forerunners, who wished to seize the Mantle of Responsibility from the Precursors instead of letting them give it to Humanity. Some Precursors hid in stasis while others changed into the dust found by the ancient humans (the latter created the Flood). The Flood attacked humanity for only a short while because the Precursors favored humanity. The Flood/Precursors found humanity worthy of the Mantle and instead traveled to take revenge against the Forerunners.
It was later revealed that the Flood was not a creation of the Precursors, but the Precursors' perverted and insane remains of themselves. The dust that humanity had discovered that led to the birth of the Flood threat was one of the failsafe measures the Precursors put into place to try and avoid their extinction. Over the Eons, however, the dust became flawed and corrupt; rather than rebirthing the Precurors as they were, it only brought a cancerous plague and death. The remaining Precursors, driven to insanity by the betrayal of their creations (the Forerunners) instead embraced the disease they had accidentally birthed, guiding it to cease infecting humanity as if they had successfully immunized themselves. The Precursors, insane, but driven by a need to create new life, decided that the Flood would be their instrument; all life they made from then on would know only suffering, and would never have the chance to throw down their makers ever again. The very act of being exposed to the Precursors minds within the Gravemind drove the Ur-Didact to a brooding, hate-filled insanity.
The Precursors' Organon—a web of information of billions of years of Precursor knowledge that is also known as the Domain by Forerunners—demonstrates Precursor technological advancement. The Organon was destroyed when the Halo Array fired, as it was an artifact of Neural Physics, which was vulnerable to the Halo Effect which destroyed all neural structures of sufficient complexity.
The basis for all Precursor technology is neural physics. The Precursors were the only known civilization to achieve Tier 0 of the Forerunner Technological Achievement Tiers and were one of the two civilizations to exceed the Forerunners in technological advancement. They can theoretically accelerate the evolution of all intelligent life and travel across galaxies. They also created the system that Forerunners used for storing information known as the Domain.
These orbital bridges, or 'star roads' were used in the final stages of the Forerunner-Flood war. They seemingly caused local space-time to warp and mutate when using their version of slipspace travel.
- 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.
- ↑ Bestiarum, page 26: Tier 0: Transsentient As the [Forerunners] had no examples of civilizations with technological accomplishment greater than themselves - with the exception of the Precursors - this is a theoretical ceiling. They can travel intergalactic and accelerate evolution of intelligent life. These may be creatures of legend.