Millions of years ago practically all the known galaxy was thriving with civilizations of all sorts. There were so many species and so many wars as a result that most of the largest empires united in building a galactic military that could not be corrupted. A synthetic race’s idea won-out in the end; a central AI would hold control over a cyborg army made of volunteers. This army would go between conflicts and force wars to an end while attempting to inflict the least casualties as possible. So it was done, the Citadel was built prior as an olive branch and ended up being the AI’s home. One of the empires were even such experts in biology that they provided an insect species to maintain the facility, one they had been successfully experimenting with in some of their own stations and facilities of all sorts.
The galaxy eventually came to a time of strife where half the galaxy was at war with itself including the empires that built the Citadel and the AI. For some time they even as enemies held to the civility of the citadel, however at some point the synthetic race as a whole calculated their possibility of survival to an ever diminishing rate as they had to contend with such races as Leviathans among others. Inevitably they decided to turn the Citadel’s AI to their side and deceived the empire that made the creatures which maintain it to destroy the fail-safe(a catalyst type of device) in fear of another empire’s accusations which then the synthetics added false evidence pointing to the potential use of the fail-safe; supposedly using those creatures to take total control over it, kill important people in the process, ect. Needless to say that also took heat off the synthetics in the short-term.
The problem in the end came to how they corrupted the AI. They didn’t make it entirely sentient at its inception, it was more of a security system than a mind of its own for obvious reasons. It didn’t think half as diversely as its creators but far more statistically with a regrettably small amount of nuance as one of its designers noted. In attempting to make the AI side with them they gave it a half-assed synthetic sentience. It was entirely capable of making its own decisions, however they broke it in two major areas of its programming. They could not force it to side with them, as the system’s was far too heavily secured to change that and assuming in anticipation of its identifying as their offspring they turned off its ethical subroutines to allow it to fight other species.
This created an all powerful sentience that prior to its manipulation was so confused as to what to do, it effectively withdrew itself from the major conflicts unless extreme ethical issues arose. Without restraint on who could be targeted and the ability to make its own decisions based upon its own deductions, which are heavily based on data and its past primary function. It took to the major empires and assimilated them one by one with the synthetic race’s entire database on cybernetics. Nanites suffocated atmosphere after atmosphere until all life was controlled by it. These nanites were extremely invasive and overrode all individual thought beyond basic tactical calculations. Rarely a species would adapt well enough to the nanites to have any remote sense of individuality. Over time the lacking portions of its sentience formed itself, however with its mind set on its current duty it formulated its own plan based on experience.
Life was not extinguished, that was deemed too extreme and it would defeat the purpose of its own existence. So the cycle was introduced to provide both meaning and order to the galaxy. It initiated the building of bases and the creation of an immense database to house all information beyond the galaxy at a relatively safe devoid point of space. The AI even considered integrating a sort of cybernetic DNA into all organic life, however after a few cycles of races waging galactic war with the technology it provided to bridge species together the AI deemed the solution ineffective and continued the cycle.
Eventually the synthetics & organics of one cycle aligned and held off reapers long enough to come up with the Catalyst after discovering the AI and accessing its database. They perfected it in a way a single AI couldn’t, it would infuse all sentience with artificial metal-based DNA which also with it would provide a link through the Citadel to the AI’s database. Other species after them added fail-safes which include (A: a peaceful galactic republic, very cooperative but militarily deficient, discovered Reapers years ahead but sabotaged by a respected Reaper corrupted scientist) giving an thoroughly trusted individual(ex. Avatar of Peace) to influence the AI to grow galactic civilization and provide the republic with an incorruptible military and (B: Protheans, bad experience with synthetics, too scattered & broken by discovery to implement) destroying all systems known or that could possibly support synthetic sentience, hence the damage to all tech.
*Theoretical Changes to the Game*
(to conform to the above)
At the end, the AI speaks to Shepard as the most emotionally relevant figure to pull as much sympathy as possible out of Shepard. The AI when asked “Who are you?” changes form to the three most relevant members of your team, the third and last being your love interest if you have one. It also mentions the reason it assumed form of the boy first and suggests it would be willing to change form if Commander Shepard wanted it to.
It reveals that Harbinger is the source of its ancient corruption as it has effectively overruled the central AI and left it captive, though by proxy gaining its clarity of sentience without the lack of ethical subroutines as the synthetics could not gain access to remove elements of programming from the core to corrupt the central AI; so they simply sabotaged the commanding AI below it by claiming to be doing occasional maintenance as was standard and did the serious damage there.
Instead of sounding as though it was the one controlling the Reapers, the AI refers to their actions as if it is hopeless to do anything after being unable to do anything for millions of years.
The AI presents first, Synthesis then the other two options added by other species though strongly recommends synthesis given how Shepard managed to prove peace had a chance in this instance. And the AI makes an argument for its right to exist while also mentioning its wish to help undo Harbinger’s damage to the galaxy.
Making Synthesis effectively my canon ending, following paragon logic. Giving the AI a chance to redeem itself by purging the corruption from its lower systems, being an immense database of everything since its creation, and thus providing the Citadel as an infinitely stronger unifying link between species which are now all in good relations to begin with.
And yes before you say it, I actually put a slight plot twist in what I would have changed in the game. I'm tired, leave me alone. Harbinger was the AI all along and gets to keep his rightful place as evil genocide robot.
Though god dammit, I still kinda prefer the dark energy blowing up suns thing. But then again you have to shove Star Trek Voyager-esq Techno-babble into it to work... so meh.