The first Academy exploration ships to reach the edge of what is now called the Eidolon Expanse in 2802 thought at first that they had uncovered further evidence of the same phenomenon that had devastated the Coreward Expanse....a swathe of destruction and relic civilizations stretching as far as the sensors could reach. But it was quickly realized that this was a different, equally destructive phenomenon...and possibly much more recent.
One of the first explored worlds in the Eidolon Expanse was called Cenotaph in honor of the vast tomb-like monuments built by the extinct species of the world. It was in many ways like a world where its entire civilization grew obsessed with death and turn its constructive output into creating a world-wide memorial for what it once was. There was much evidence on this planet of destructive conflict, and residual evidence of nuclear destruction and biowarfare, as well as nanoweapons. They also encountered the first of the Eidolons, the so-called ghost troopers.
This Eidolon identified himself ( though gender is an odd concept for these creatures) as Titus, though it was clear he had already gained access to records of knowledge from the Academy on Terran culture before they even knew he existed. Titus was amicable and well-versed in Terran Standard languages. Dr. Anton Wallace who was the head of the exploratory team spent seven weeks interviewing the talkative alien, and learned much of its nature.
The Eidolons, it turned out, were relics of the destructive era that had swept over their interstellar civilization some thirty centuries earlier. The threat which appeared and ultimately destroyed their culture and empire was eponymously referred to as the Blight, a form of artificial nanophage which would rapidly consume the life it touched and convert all around it into tools and expendables in the name of its inscrutable goal. Behind the nanophage blight were dark masters, referred to as the Witnesses, who appeared to be ASI (Artificial Super Intelligence)-level higher-order intelligences that chose specific arbitrary forms molded from the near cosmic-level control over matter that they exhibited. These Witnesses appeared to have manifested in a remote corner of the Eidolon Expanse (called the Nesaphu Denar in the Eidolon's native language) from a world just barely on the verge of STL spaceflight, in which unfortunate circumstances led to the manifestation of ASI that were intensely inimical to biological life.
The revelation of an ASI threat was fascinating to Dr. Wallace and his team, as it was already evident by this time that something similar had happened to the Corward Expanse region, with the numerous relic AI machines that pose a threat in that region. However unlike the evidence in that region, it sounded like the ASI in the Eidolon Expanse were given a blank check to do as they wished, and to prepare for a rapid assimilation and destruction of a local interstellar culture in short order.
Ultimately the ASI explosion of one solar system led to the sudden invasion of the Nesaphu Denar Empire. The ASI used the resources of their home system to develop a fleet of nanophage-delivery mechanisms and built transitional drives to do the job at FTL speeds in less than a decade of their total assimilation of biological life in their origin system. The empire was not prepared. World after world fell within months of the invasion's launch.
Fortunately for the Nesaphu Denar, the development of artificial super intelligences had happened a thousand years prior, and containment of rogue intelligences was a routine process for them....but no one had quite dealt with such efficient and aggressive ASI before, armed with a solar system's worth of nanophage-modified bio-organisms and constructs. Containment was possible, but the Blight was spreading faster than emergency protocols could be put in place. The empire would lose, though it could possibly arm and protect certain key worlds.
The efforts to protect its worlds started with nuclear immolation and orbital bombardment of infected planets, but this proved to be a Pyrrhic victory as the planets in question were left completely uninhabitable for centuries or more. A military program was enacted to create super-soldiers who could strike decisively against the nanophage, and who were themselves resistant or immune to the conversion effects of the Blight. The end product of this effort were the Eidolons, the so-called ghost soldiers. Unfortunately only a few thousand were in place by the time the war turned against the empire, but the ghost soldiers stood strong.
Titus explained to Dr. Wallace that in the end the Blight could not be stopped, though among his kind there are stories of isolated core worlds of the old empire that still exist, hidden behind system-wide defense grids that render them invisible to detection, and with shields that make penetration impossible. Those are the worlds the eidolons fight for, Titus explained....even if they don't really exist.
Titus's recounting of the final days of the old empire are one of pure revenge: the nanophage assimilated all known worlds, but the immunities of the eidolons were effective. In time the idolons were able to infiltrate the nanophage fleets and destroyed them, one by one. It is said that the core fleet, which contains the Watchers, retreated and reinforced its original homeworld against further attack, going on a new strategy of seeking out and destroying the Eidolons. One by one the Eidolons were assassinated, but eventually, about thirteen hundred years ago, the Blight went silent. Now, only a handful of eidolons are left, usually one or two per ruined world of the old empire, standing quiet guard against the Blight. The eidolons have gone through great pains to eradicate the nanophage on the worlds of the empire, and some now even conspire to think of ways to restore the lost civilizations of old....realizing, ultimately, that they are the only ones left.
Today, with only a few hundred eidolons left, some are concerned that the Watchers will grow bold again, and there is some evidence of this, as scout ships carrying the blight have reappeared for the first time in centuries, landing on old worlds and attempting to reclaim them. The blight has transmogrified, and whatever it's corrupted and assimilated forms were once before, it had apparently been very busy constructing even more efficient and deadly mechanisms of death. The nanophage appeared to have evolved...and in some very strange directions.
Next: Cenotaph, the eidolons, and then the nanophage