Hello again ladies and gentlemen! It's been a while since the last time I wrote a Mass Effect fanfiction. At this moment, I'd like to test my lore-expanding ability in writing fanfiction, and I think I'll start with making a non-canon "precursor"-like species predating the Reapers. Rate and review my story if you find it great. Enjoy!
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Legends of a powerful creator race or species are not something new, even to the Citadel species. Although the names, motives, and origins of these legends are vary, they always share the same plot and outline; ancient entities whose existence predate the known world as we know it. The entities are supposedly the creators of the galaxy (or universe) with knowledge and power unimaginable to 'mortals', so we speak, living somewhere in the deep space, or hidden inside their distant sanctuary, leaving nothing about their culture or past interference but the rarest, scantiest of relics.
In a scientific light, this precursor/forerunner race is classified into pseudo-science matter, especially with the contemptible claims of them being a species much older than the Protheans. With no evidence of the supposedly pioneer species ever found, the mainstream scientists, Citadel's best scientists to be exact, refused to study the subject any further. This account seems to only fuel the faith in the precursor's existence, coupled with an large quantity of "sightings" reported throughout the galaxy.
The Asari, believe it or not, kept a large amount of recorded mythology about "Athame's Chariot", which tells an alleged race of deities from which the Asari's patron goddess, Athame, descended. The University of Armali, Thessia, performed a number of researches on the tales' origin for ten years or so before Thessia's government declared that the university's research as "a waste of time". In the end, all of the records were simply left in libraries across Asari space.
Other races also share the same kind of "precursor myth". The Turians, with their "Spirits of Palaven's Sky" tales, the Salarians with "Hunters of the Star", and even the Hanar with their "The Enkindler's Progenitors" being a lesser-known sect of their Prothean-based belief.
Of course, not all galactic cultures speak of them in a godly or saintly way. Some, like the Batarians, tell mythological stories of "Great Mountain Beasts" who once created the world from rage and madness, creepy monsters who hid themselves deep within the Batarians' home planet, Khar'shan, so that when the time comes, the Mountain Beasts will come out and rule the world. Another good example is the Volus, in which their mythology and fairytales spoke of a precursor race called "Shadows of Dark Skies", a group of mysterious beings perhaps similar in theme with H.P. Lovecraft's Cthulu mythos.
Even the Krogan race, in their early days, told tales of a forerunning race named "The Maw Riders". There is no surviving record of this mythology after the Krogan Rebellions which rendered every known archive on Tuchanka in ruins.
Attempts to uncover the secrets of the "forerunners" usually ended up in failure. The most common reason (let's say, excuse) being that the purposed search location being located beyond dormant mass relays which, thanks to Citadel Convention, is strictly forbidden. Either because the Council doesn't want to risk another potential galactic war or, as conspiracy theorists argued, unwilling to discover something so controversial it could shift the galactic clout everyone so adapted to. Some of these dormant relays are located within the Terminus Systems, which is full of pirates and slavers, further adding the impediments of those willing to expose the forerunners' secrets.
To top it all, only a tiny number of books dedicated in solving this mystery has ever been published. The books, according to those who bought and study them, seems to mostly contain unverified evidences about the "precursors", and oftentimes written by controversial people not acquainted to any researchers acknowledged by either the Citadel Council or trustworthy scientific organization.
The currently-elected Asari councilor, when inquired regarding the stories, simply answered "The Council and its top scientists and scholars do not support the existence of these so-called 'precursors', unless a proper evidence can be retrieved and properly studied by Citadel scientists." Despite her statement, many believed the truth is still hidden out there.