According to legends passed to the present day by various ancient xenosophont races, the Dawn Hunters were ancient, alien, semi-mythological self-replicating autowars with godtech level technology. There is no certainty that the Dawn Hunters ever existed, or whether they still exist. According to folklore they were built by unknown creators towards the galactic core (in regions beyond terragen-colonised space) several tens of millions of years ago, implementing whatever rules for interstellar conduct the creators saw good and then spread across space.
What evidence there is of the Dawn Hunters is derived from exopaleontological and xenoarcheological research, the various hints being pieced together to make this very tentative account. There is serious doubt that the Dawn Hunters even existed, as no ship hulls have been discovered. However, it does seem that there were destructive ancient autowars and nanoswarms. According to some these have been conflated into a single "Dawn Hunter" hypothesis; others do see a true connection. For example the robot war machines that attacked the spacefaring Muuh some 4.9 million years ago, and the sudden disappearance of the flourishing civilisation, at approximately the same time, may indicate something, with proponents of the hypothesis explaining the apparent inconsistencies with Muuh accounts as due more to differences in psychology and perception between the two races.
The original purpose of the Dawn Hunters - according to proponents of the Dawn Hunters scenario - was to protect the universe from threats like out of control nanoswarms, but they also had a more sinister motive as well. The Ships would scout the galaxy, spreading out and wipe out empires that created these swarms. Whether they would warn beforehand or even have a rational system of law didn't matter.
Since they disallowed any competing systems, attacked anyone that attacked them, and in general counteracted hegemonizing swarms, they went unchallenged. The Ships spread out over a wide region of interstellar space (the boundaries have not yet been determined), scanning for crimes against their laws. Once they detected suspicious activity the ship investigated and if it judged a crime had been committed it began to multiply, possibly signalling other nearby ships, and attacked using godtech weapons.
Legend has it that there were three distinct clades of Dawn Hunters. Those who believe the Dawn Hunters hypothesis have named these the "logics", the "aesthetics" and the "enigmas". Each is supposed to be at war against the other two but unable to defeat them. At the height of their "empire" the galaxy - or at least a region there-of - was divided into three regions, one ruled by the Aesthetic Directive, one by the Platonic Directive and one by the Enigma. The Aesthetic Directive apparently valued complexity, so it would fiercely protect natural planets from being wiped out and harass/destroy static or diversity-decreasing empires. The Platonic Directive seemed to show similarities with Keterism and Negentropism, possibly favoring the emergence of Jupiter Brains, efficient energy management and the advance towards transcendence, and destroyed "failed" civilisations or those spreading entropy. Enigmas were never understood by any known species; the most popular theory is currently that they were actually a kind of joke by their creators.
The social result of this was that the subservient clades came together to exchange information about the "laws" so that they could avoid being wiped out. Exoarcheologists studied the remains of executed civilisations, theoreticians tried to map the laws and so on. It seems that there was an intense quasi-darwinian selection process among the Dawn Hunter clades: weaker designs were defeated and vanished, while the survivors are roughly on par or did not fight each other. Reference is made to new laws arriving from the galactic halo, most notably certain globular clusters, and being repulsed. Rather like the Sephirotic Empires today, each system carved out a broad swathe of interstellar space. They do not appear to have ever developed stargate technology or used exotic matter for the creation of wormholes or reactionless drive; the reason for this is unclear. Where the empires met (towards the galactic core) a continual war against each other ensured, and may possibly still continue, making that region extremely dangerous.
It is suggested that, as with the present sephirotics, all Dawn Hunter clades (assuming they really did exist) were derived ultimately from a single ancestral civilisation that has transcended to a higher toposophic. It is not known what happened to the originals or their home system. Possibly they were destroyed by their descendant clade, or that they uploaded their consciousness into the clade's datasphere. The reason for the disappearance of these once invincible forces is another enigma. Theories of course abound. The "shared undermining" thesis argues that accumulated knowledge of loopholes and ways of dealing with the Dawn Hunter clades among the extant civilisations undermined their position. Another theory suggests that eventually more advanced forms of weaponry or counter Dawn Hunter clades fielded by advanced civilisations ended their rule. There are even tantalizing hints that the Dawn Hunters largely self-destructed for unknown reasons. Still, there is no way of knowing if all the Dawn Hunters are gone, or if they are just scarce in the current era.
As with all mysteries, a huge body of myth, legend, and folklore has developed around the Dawn Hunters. In colonies like Aetherius Orbital and the nearby Adamski Habitat belief in "greys", "Dawn Hunters", and "dragonoids" is standard. According to the teachings of the so-called "Lawgiver" sect (popular in some worlds of the Inner Cygnus region) the Dawn Hunters are simply in hibernation, but as terragen influence expands towards the galactic core they will re-active and once again reassert their dominance over the galaxy.
Another aspect of the Dawn Hunter hypothesis/myth is the observed fact that many worlds which seem to have had quite large populations at one time appear to have been cleaansed of all (or almost all) evidence of their presence. This cleansing is attributed to the so-called Restorers, a hypothetical set of entities who may have been associated with, or in opposition to, the Dawn Hunters.
Mythopoeologists have long pointed out that this hint of Dawn Hunters in unexplored space, like the "there be dragons" on the edge of the ancient Age of Sail maps from Old Earth, elevates the status of Dawn Hunters away from hard science and into the realm of mythology.
A popular sceptical explanation is that what physical evidence there is regarding the Dawn Hunters and other inhibitor relics points not to a mystery godtech supercivilization, but relatively primitive (by terragen standards) autowar swarms that have accidentally killed their originating civilisation, but continue to present a problem to subsequent sophonts. One candidate for this event is the release of destructive autowars by the unnamed civilisation HIE636MZE, 196 million years ago. More recently the Enforcer drones of unnamed species: HIE072CZE have presented a threat to explorers, but they are relatively easy to deal with and are unlikely to be the origin for the Hunter mythos.
At present the main centers of research into the Dawn Hunters is the Exopaleoarcheological Institute - headquarters KoHold City, Aw Addia (third planet, Van de Bos' Star), the Hamilton Institute of Exopaleontology, Delta Upali C/D II, and the Center for Galactic Folklore and Mythopoesis, Gamma Hyeidon IV, and the New Jung Mythopoetics of Abraxas Station.