The study of parallel worlds is one of the more obscure and misunderstood schools of magecraft. Outside of those with some connection to Zelretch's Second Magic, there were few who thought about the concept on anything deeper than a purely theoretical level. Had you walked up to any random member of the Clock Tower and asked them to explain the Primary Branch (the central cluster of parallel universes supported by The World,) or the Lostbelt (parallel worlds that have diverged so much from the Primary Branch that they have become completely separated from it,) you would likely receive a blank stare and a quick excuse to leave.
As such, rumors that a rare fluctuation in causality would cause several distant Lostbelts to be temporarily connected to the Primary Branch were treated by most as a simple curiosity. A few more inquisitive mages would attempt some radical experiments during this window of opportunity, but for most of the Association the event would pass by without any significant impact on their lives.
That is, unless they were participating in a certain bloody ritual that took place every 60 years in a small eastern city. Decades of careful planning and preparation for the Holy Grail War were thrown into doubt by one simple change: the Servants who would be summoned to participate in the war would not be those from the well known History of Man, but instead heroes from the unknown Lostbelts.
(*)
It was a simple matter of mathematics.
Ten human lives were worth more than one. A hundred lives were greater than ten. A thousand greater than a hundred. That simple algebra was the core behind the concept of justice. If given a choice between sacrificing a single loved one or hundreds of other humans, there was only one correct solution.
A young Emiya Kiritsugu repeated that thought for the hundredth time as he stopped his motorboat just outside of New York City, directly in the path that Natalia's plane would take. He repeated it another time as he said his last words to the woman he had considered his mother and fired the Stinger missile. And he repeated it once again as he cried in grief as the fiery wreckage of the plane crashed into the water.
This was the correct choice. Despite his pain, Kiritsugu held to his decision. The many always were greater than the few. That was justice. That was the only way to peace. And Kiritsugu would commit any evil or sin conceivable in order to achieve peace.
(*)
It was a simple matter of mathematics
Human lives were not equal in value. People may claim that they treated everyone equally, but only a truly broken person could actually go through with that claim. Everyone had someone they considered more important than anyone else. If given a choice between sacrificing a hundred meaningless strangers or a single loved one, there was only one correct decision.
The Foreigner stood in the burning ruins of the city, their hands drenched in fresh blood. High above, cracks spread across the sickly green sky. These were the last dying moments of this world. Another seven billion lives thrown into the pyre.
And it still wasn't enough.
The Foreigner's screams of rage echoed through the hollow buildings. How much longer would it have to fight? How many more sacrifices would need to be made? How many more worlds would they need to destroy?
The numbers didn't matter in the end. Any cost was worth it to save the one they loved. They would sacrifice everything that lived in exchange for this one person. No price was too high, no method too wicked, for the opportunity to spend just a few more minutes with them.
The sky finally shattered completely as this world began to crumble away. It was time to leave for another universe. One which just might hold the secret that the Foreigner had been chasing for all these aeons.
As the last remnants of that world faded into nothing, the Foreigner whispered a name. Just one word, but filled with immeasurably grief and longing. That name echoed across the broken world as it finally fell apart into oblivion.
Author's Note
Since this is not a traditional crossover, I felt it necessary to explain a little about the idea behind this story. Rather than drawing on other popular media franchises for characters, I'm using characters from pieces of original fiction that I've written. The whole idea for this started when, during a moment of boredom, I decided to take some of my favorite characters and figure out what Servant class they would fit into. Next thing I knew I was giving them Servant parameters, then I was considering what would happen if these people were thrown into a Grail War, and all of the sudden I found myself outlining a fanfiction of Fate/Zero with all the Servants replaced by my characters.
I'm writing this fanfic with the assumption that most of its readers won't be familiar with the background for any of these stories, as only one or two of them got any popularity outside of small writing communities. So you don't need to worry about scrounging up and reading through all of the background to understand what's going on; I'll include all relevant information in this fic. If you are curious about where these people are coming from, here's a short list:
The Empire of Omegia (Saber, Lancer, Assassin): Post-apocalyptic sci-fi/fantasy setting based heavily on Gnostic mythology
Mephi (Archer): A sci-fi-time-travel-action-romance-thriller thing with several different Lovecraftian Mythos all thrown into a blender and set to puree
Smiting the Gods (Berserker): My old Slender Man story from way back when I was a young and fresh-faced undergrad. Mix between gothpunk action and horror, leaning more towards the action side.
Chaos Rider (Rider): Dieselpunk action/thriller with inspiration from Japanese Tokusatsu shows. So far only a prologue, titled The Soulless Empress, has been written and published online.
The Queen of Maggots (Caster): A short horror story using elements from the open-source Fear Mythos
And that's all for the intro. Time to get back to the story y'all are actually here for.