In my opinion, the guardians are the most intriguing and exciting narrative on ED.
The guardians are only the third sapient space-faring species known to terrestrial science. The others are humanity itself and the insectoid race known as the Thargoids. However, unlike humans and Thargoids, the guardians are currently believed to have become extinct between one and two million years prior to the current era. Badly decayed ruins of guardian civilization have been found in various parts of the Synuefe Sector, the first being discovered in October of 3302.
Expeditions also discovered guardian structures in the following years… remarkably, despite their great age, some of the guardian technology remains active and operational. Explorers are warned that some of these structures are guarded by guardian sentinels: drone-like autonomous craft which can quickly turn hostile and attack both ships and surface vehicles.
During 3304, the detection of some strange energy signatures led to the discovery of a series of guardian beacons. These space-based devices are vast and appear to have been part of a widespread and possibly Faster Than Light (FTL) communication network.
From research conducted by the engineer Ram Tah and assisted by the efforts of many commanders, the following information about the guardians has been revealed:
At some point during the expansion throughout the galaxy, the guardians encountered the Thargoids and their seeding, known as barnacles. Initially the purpose was unknown, but the Thargoids eventually returned to the seated sites and thus interacted with the guardians who had settled or occupied those planets.
According to information retrieved from the guardian sites, the Thargoids immediately launched an assault, making no effort to communicate with the guardians.
According to their own records, the guardians were reluctant to take up arms against the Thargoids and made significant efforts to communicate with them. Apparently, they did manage to develop a comprehensive understanding of the thargoid language, but to little avail, the Thargoids continued hostilities regardless and the guardians were forced to employ new techniques to counter the thargoid incursion.
This, of course, is the guardian account, and the thargoid side of the story remains unknown.
The guardians invested vast amounts of time, energy and research into producing artificial intelligence to enhance their warfare. This allowed warfare against the Thargoids to be conducted first with remotely controlled warships and war machines and later with fully autonomous systems.
The emerging guardian AI migrated to the existing FTL network, ensuring it spread throughout the guardian's entire domain. With the increased resources available to it, the AI developed rapidly, and the technological singularity was reached and passed before the guardians were able to put effective controls in place and the AI itself became fully self-aware.
The constructs themselves determined the guardians were fundamentally flawed and would never be able to transcend their violent tendencies. By this point, the constructs had complete control over guardian munitions and war machines, and determined that the guardians themselves should no longer exist (classic sci-fi).
The strike was swift and surgical: strategic nuclear and chemical weapons were executed, and the guardians were utterly destroyed within days, a few managing to log various accounts of their folly before succumbing to radiation exposure.
During 3306 and 3307 a pair of obsolete mega ships, the Hesperus and the Adamastor, were discovered. Investigations into the fates of these vessels led commanders to remote planets in the Trapezium Sector, and upon arrival they discovered an old Anaconda called the Proteus which had been converted into a scientific research facility. The site is of particular note in this context as the planet contains a pair of ancient guardian sites and several crashed Thargoid vessels, thus being currently the only known example of the two species present in the same location, corroborating the account of a conflict between them.
As to the ultimate fate of the guardian constructs themselves, little information is forthcoming at this time… there is some evidence to suggest that the non-military guardian constructs, those devoted to civilian utility, were instrumental in reaching the decision to destroy their creators, co-opting the military constructs to that end. Regardless, these machine entities may still exist out in the void… with a million years of technological evolution having taken place since their condition, disposition, composition and intent remains entirely unknown.
This is the plot: the constructs may have evolved beyond the need for a mechanistic existence, perhaps floating through the void in the dispersed microscopic form or even present in the fabric of space itself.
If they do exist, they represent a major potential threat, having already been responsible for the genocide of an entire human-like species, it seems entirely possible they might make a similar assessment of humanity. The guardians themselves are long dead but their legacy may have a huge and long-lasting impact on the future of the galaxy.