Disclaimer: I don't own the Fate franchise (obviously). If I did, we would've gotten Fairy Knights for Agravain and Mordred.
Fate was a funny thing.
It was intrinsic in the shaping of lives, species, worlds and timelines. Divine Spirits who held authority over it were amongst some of the most feared of their pantheons and even the souls of the exceptional dead were subjected to it's whims, frequently forced to replay the events that they took part in in life when summoned back to the living world.
Which made the rare cases in which it was broken all the more interesting, as was soon to be the case of one timeline's Fifth Holy Grail War.
It was over Fuyuki City within the metaphysical plane that the mass of magical energy known as the Greater Grail resided. Within that mass was the being whoo had once been known as the Avenger Angra Mainyu. And Angra Mainyu was, for lack of a better term, reprogramming.
If All the World's Evils could be described as an exceptionally nasty computer virus, then the Greater Grail could effectively be compared to an exceptional supercomputer with powerful firewalls and multiple in-built safety features in the case that it was infected. Both of which had failed.
With the Grail's failure to successfully defend itself, Angra Mainyu now had complete reign of it and had taken delight in toying with it's functions. The feature that only allowed for heroes to be summoned? Gone; anti-heroes and villains could now be summoned as Servants. The feature that only allowed for Western heroes to be summoned? It was taking a bit of time but it would most likely have something figured out by the the time the Fifth Grail War occurred within a few weeks time. The feature that prevented Servants from past a certain time period being summoned... wait a moment.
Angra Mainyu blinked in confusion (metaphorically that is) as, for the first time in the decades it had been inside the Greater Grail, it noticed that failsafe. Taking a moment to examine it, Angra Mainyu had to admit that it was cleverly hidden; by placing the multitudes of other, flashier failsafes and restrictions in front of it, it had been easy to overlook and miss. It was only by mere chance that they had missed it all these years, having not deemed it important enough to look passed the other failsafes.
A grin stretched across Angra Mainyu's face (metaphorically, again) as it began tearing it down with all the gusto of a kid being given an entire birthday cake all to themselves. The thought of why the failsafe was even there briefly crossed their mind before tossing said though aside to revel in the further corruption of the grail.
Within minutes, the failsafe was in shambles and shortly after destroyed. Angra Mainyu basked in the satisfaction of it's destruction before it took notice of something: the Greater Grail was now absorbing mana from the leylines of Fuyuki at a rapid pace and stockpiling it within seven hastily-constructed separate reserves that were quickly overflowing into and drowning out the rest of the Holy Grail.
If Angra Mainyu had a face at the moment, it's expression would have turned into one that conveyed the message of "oh fuck" before they were swept away in a torrent of mana, dragged off to elsewhere in the grail by the veritable tsunami of magical energy.
As that took place within the Greater Grail, a resulting chain of events took place in a different metaphysical plane. Within the Throne of Heroes, ancient souls long dormant were being roused from their slumbers, awoken by the beacon into the living world that Angra Mainyu had unknowingly created. They saw a chance that shouldn't have been possible, to walk the world yet again.
And they fought for it, tooth and nail.
The olden souls of heroes of history, myth and legend, whom had been dragged closer to the Grail's metaphysical location to answer their upcoming summons, unconsciously fought for their very right to be called upon for this Grail War against those more ancient souls, colliding against one another for the chance that their wants and wishes might be realized. They clashed unknowingly, unconsciously dreaming of battles that could never be as they pushed one another aside to claim their spots in the Fifth Holy Grail War.
An ill-tempered survivor maneuvered their way between the dreams of others without being noticed.
A blue-haired spearman dreamt of battle against a mountainous beast who shrugged off everything he threw at it.
A horned figure materialized it's Mystery against a beautiful, blindfolded warrior woman.
A horde of young beasts fought alongside two enemies to combat a beautiful witch.
And the strongest there was fought without restraint against the queen of a world long dead.
By all accounts, the heroes should have won their battles against the ancient souls, even if not without great effort for some of them. Each of their names were known throughout the world while the ancient souls had little renown to their names. The heroes bore the crystallizations of myth and worship; legends given form and name while their opponents bore nameless facts and speculations. The heroes fought with all their conviction to see their wishes through while the ancient souls fought merely to live.
And it was that last fact that allowed the ancient souls to win.
For as the Daemon King and Last Master of another timeline could attest to, the desire to live could even overcome the ambitions of demons.
The survivor quietly reached the beacon without being dragged into conflict.
The body of the spearman was thrown into the distance, broken by the now-bloodied mountain-beast's tail.
The multitudes of the horned one's Mysteries forced the warrior woman to abandon further combat.
The witch abandoned her fight in annoyance after coming closer to defeat than she cared for by the horde and two enemies.
And the strongest there was met defeat at the jaws of the queen.
With the dreams having reached their ends, the souls of Cú Chulainn, Medusa, Medea and Heracles quietly drifted away from the ancient souls and back to their own places within the Throne of Heroes. While their battles may not have left a lasting impression on those souls, as was the nature of dreams, the ancient souls had unknowingly rejected and destroyed what would have been the fated outcome of the Fifth Holy Grail War.
No more would the boy who wanted to be a Hero of Justice summon a blonde Saber. The survivor refused to yield it's right to live to anyone else.
A fated betrayal would never take place. The mountain-beast's presence would make sure of that.
The adopted heiress of the Matou family would not become a Master. The horned one had it's own plans.
The strongest of all would not become the fairy-like girl Servant. That right belonged to the queen.
This would not be a Grail War between the likes of heroes and villains of old, as fate would have made it to be. This would be a war between survivors, desperately trying to live once more after having been roused from their slumbers spanning eons. This would be a more ancient grail war.
Author's Note: I tried to make everything seem like it could really happen within the Nasuverse but I don't know if I did that very well. If someone who reads this is well-versed in the Nasuverse lore and notices that I messed something up, please let me know.