The Precursors were an advanced race that preceded and were mythologized by the Forerunners. The Forerunners classified them as "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 scattered across not only the galaxy but also outside it, namely in the Large Magellanic Cloud.
Due to the Precursors' transsentient nature, their true physical form (if they took one) is unknown. According to the Gravemind, they could assume any physical or immaterial form they saw fit, having done so over billions of years of existence.
At least three forms are known with some degree of detail. One form of Precursor, or at least an infantile stage of them, took the form of green teardrop plants as large as melons that when exposed to light would unfurl wide leathery leaves that would fan gently with the grace of a bowing dancer, and expose a glowing starburst of delicate white blooms dangling from groups of lantern-like stems.
Another form of Precursor is the Primordial who was massive in size; the mold encapsulating the being on Charum Hakkor outlined a being approximately fifteen meters tall and eleven meters wide with the Ur-Didact seeing it on Charum Hakkor describing it as possessing an insectoid head, four arms, and two degenerate legs and a long, segmented "tail" being attached to the base of the skull and tipped with a two meter-long barb. Each of its hands had three fingers and a central opposable thumb. The face of the Primordial had been likened to that of a eurypterid, or "sea scorpion," a creature that the Precursors had allegedly seeded on a number of planets including Earth. Its head was flattened, with oval compound eyes and insect-like mouthparts. Later, aboard Installation 07, the being was described as having a large number of legs which it held in a curled-up position similar to a spider. The creature's torso was described as "grossly fat," and its skin was covered in a glassy, crystalline coating; a fine powder often fell from its body, perhaps Flood spores or the same powder responsible for the original Flood outbreak. As a Gravemind, the Primordial was capable of shifting its physical form to an extent, such as rearranging its limbs. However given that the Primordial was said to be heavily mutated to survive vast spans of time, it is likely it no longer resembled other Precursors as they appeared during the time of the Forerunner-Precursor war thus it is likely this form was unique to the Primordial.
The most detailed and most prominent form of the Precursors known to the inhabitants of the Milky Way is their form as the Flood, a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient lifeforms of sufficient biomass and cognitive capability. This form was initially born of corrupted Precursor remains.
Precursor technology was incomprehensibly advanced, to the point that the Forerunners labeled it Tier 0, or "Transsentient". Precursors based their technology upon what Forerunners called neural physics: the concept that inanimate matter and thought are inextricably linked and that the universe itself is effectively a living entity. By drawing from the fundamental energies of the universe, the Precursors built exotic, virtually indestructible architecture and traveled across interstellar distances using a superluminal transit mechanism based upon the same principle. It was believed by Forerunners that the Precursors also used neural physics techniques to construct artificial planets and star systems, with Charum Hakkor believed to be one such planet. The Precursors traveled between worlds in vessels that used incredibly advanced artificial intelligence constructs known as knowledge engines and walked upon star roads. The Precursors created stasis capsules to contain beings within stasis. The Halo Array, being neurological weapons, are one of the only ways to destroy or otherwise damage Precursor artifacts. This was first evidenced when Charum Hakkor was used as a testbed for one of the Halos: the weapon's firing shattered every Precursor artifact on the planet. This also explains the lack of any Precursor ruins in modern society, as most, if not all, were destroyed when the Halo Array fired and sterilized the galaxy at the end of the Forerunner-Flood war.
Some Precursor artifacts were known to precede the Forerunners by hundreds of millions or even billions of years and, being impervious to conventional forms of damage, were durable enough to survive being 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. In truth, the Organon was a Precursor knowledge engine known as Abaddon, who served as the overseer and guardian of the Domain. Although incorporeal, Abaddon was capable of interacting with the physical world in various ways, even vaporizing corporeal beings at will.