Idea has been in my head for some time and I decided to get it out of the way by writing it.
Crossover of Harry Potter and Fate/Stay Night, neither of which I own.
30th July 1987
In the library of Little Whinging, there was an old man seated at a table, drawing the attention of the few inhabitants present in the place. With spiky grey hair and a beard, he looked every bit the distinguished gentleman, especially with his formal-looking clothing and cape, and the white gloves on his hand. He was clearly busy; looking at the library book, he would scribble across a piece of paper, as though writing down what mysterious new discovery he had made from the book.
Yet it seemed that his work was hardly serious, more of a flight of fancy than anything else. For eventually, he would get up and leave, slotting the papers he had been writing on into a random book and leaving it on a shelf in the library.
When the librarian approached him about becoming a member, he refused, replying, "I am simply passing through. I doubt I shall ever come back again."
As he left the place, the Dead Apostle Ancestor known as Kischur Zelretch Schweinorg was passed by a thin boy with black hair and spectacles in extremely baggy clothing. Despite the young lad being of no concern to one as old as he, and Zelretch would leave this world through the magic of the Kaleidoscope, with a small smirk on his face.
Six year old Harry Potter had come to the library to flee from his cousin Dudley and his gang, and their usual game of Harry Hunting. Ever since he found out that they didn't want to spend time in the library, it had become a haven of sorts to him, and on some occasions, a route of escape from the world that he found himself in through the means of the storybooks around the place.
As Harry walked down the aisles, he noticed a book with papers sticking out haphazardly, and thinking it was simply loose pages, he took it down to arrange it in order, having been ingrained to do so by his Aunt Petunia's obsession with orderliness. That book, was the same book that was left by the man he had passed by on his way in.
He soon realized that the papers did not match the book at all, and removed them from the tome. A brief jolt of curiosity that survived his relatives' upbringing made him look at them, and he tried to comprehend the information written down.
'A Ritual to Summon a Heroic Spirit.' was the title at the top of the front page, with a lengthy description of the steps involved.
Young Harry Potter often read about heroes, people who did great things, rescuing damsels in distress, beloved by everyone. He often imagined that a hero would come and rescue him from his cruel relatives, when he was locked inside his cupboard with nothing to do. Harry wondered if this was true; was magic real, and if he followed the steps involved, would a hero come to save him?
Caught up in his daydreams, Harry started to read through the steps, memorizing the circle and the chant as best as he could. His finger would trace the design over and over, and his mouth formed the words silently, pretending that they did work. Even if it wasn't real, at least his imagination would help him to feel better.
But all too soon, his time of escape came to an end, and Harry trudged back to the house where he stayed to carry out his chores once more.
"...!" Raised voices caught his attention, and Harry saw that his cousin and his gang of friends were bullying a young girl.
His mind thought about the situation. If he interfered, they would target him, and then Dudley would blame him to his aunt and uncle, and he would get locked inside his cupboard again. Better he stay out of sight entirely, and be uninvolved at all.
Except... the thought of what being a hero meant came up in his thoughts, and Harry acted before he knew what he was doing. Perhaps deep down, he hoped for the love and acceptance that the hero would attain after his chivalrous deed, or maybe he simply wanted to do what was right.
"Leave her alone!" Harry shouted, pushing his way in to stand in front of the girl.
As the first fist came his way, Harry wondered for the first time if heroes actually existed at all.
The outcome was just as he expected. He was beaten up and left aching outside, crawled back to the house only to be blamed, beaten by his uncle and thrown into his cupboard without any food. Harry even heard something crack in his chest and right arm during the beating, and he tasted a coppery flavor on his tongue. For many hours, he just lay there, stunned, until his relatives had all gone to bed.
Blood... I'm bleeding... Half crying, half laughing, all too soft to be overheard in the darkness of his cupboard, Harry pulled up his hand-me-down shirt from Dudley to prod at the pain in his chest, wincing as his finger makes contact. A hero... I was beaten up... that girl ran off... I...
Dully, he touched his mouth, picking up blood on the tip of his finger. He would draw the circle, and say the chant. If anything, it would make him feel better at least. His dreams always helped him escape from life at Privet Drive.
Biting his lip, Harry scanned around for a place to draw the design, and eventually settled on his chest. Sitting up hurt too much, and he couldn't draw comfortably on the walls lying down.
Using his blood, he traced the circle, replenishing the blood from his mouth whenever the feeling under his finger changed. Bit by bit he added to it, including his own additions to make up the parts he could not remember; inside the six-pointed star, his random tracing formed a seven-pointed one, the border of the circle gained random designs that were replications of things he liked. And to top it off, he wrote his name, a child's cry to be known as something other than just 'Boy'.
Resting his palms on the rough circle on his chest, Harry tried to recall the words to the ritual. He couldn't recall the whole thing, but he just tried it anyway.
"I command thee..." Harry coughed, his breathing slightly off, "Thou shall come to... me..."
He could almost imagine the circle starting to glow. "Thy sword shall control... my fate... Abi-, abiding by the..." The boy tried to remember what came next. Something that starts with G... "Gate of... Truth's... law..."
"If thou..." Another strange word that Harry couldn't remember. "Follows this will... and reason... then answer me... this..." Harry coughed again, "Oath sworn here..."
Part of Harry wondered if his eyes were bleeding; the inside of the cupboard seemed to be a dull red instead of its usual dark shadows.
"I... will be the... embodiment... of good... in this world..." Was that lightning? But it's red...
"I... will... dispose of... the evil... in this world..." Harry wondered if he was seeing and hearing things now; the red glow, the crackling noise... would his uncle hear and come down to punish him?
The next line failed to come into his mind at all, and he quickly cast around for what would fit this oath. Something that rhymed with seven... that's it! "Seven heavens that... bear the... trinity... come forth..." In or out? Can't remember... "Into this circle of... constraint..."
Everything seemed to be shaking now. Harry closed his eyes and spoke the last line, "Heed me... and my fate... shall become... your sword!"
Outside his cupboard, the clock hit midnight at the same time the incantation was finished. For a moment, the light and sound vanished, and Harry could hear his uncle shouting something; perhaps it was all just his imagination after all...
Then his world exploded into blinding white agony, and his screams woke up the whole of Privet Drive.
In a realm of empty white, where the only thing of note was a giant gate of stone, a BEING of pure white sat before the gate, looking at the design on it. The BEING suddenly stiffened, and turned around.
THIS IS NEW. SOMEONE FROM ANOTHER WORLD PAYING THE TOLL?
WHAT ARE YOU OFFERING... THAT UGLY THING? CERTAINLY, I CAN ACCEPT THAT AS PART OF THE TOLL, BUT IT'S ONLY A PORTION OF A WHOLE. AND IT IS HARDLY ENOUGH TO PAY FOR WHAT YOU ASK.
YOU HAVE NO IDEA WHAT IT IS, DO YOU? BUT NEVER MIND... YOUR-
The BEING suddenly paused, and turns to look at the gate, which had creaked open a little.
WELL, I'VE NEVER... THIS IS THE FIRST TIME THAT THE PAYMENT COMES FROM THE OTHER SIDE.
It turns back to face its conversation partner, a too-wide grin spreads over the place where a mouth would be on a human.
IN EXCHANGE FOR THAT THING AND YOUR APPENDIX... YOUR WISH... SHALL BE GRANTED.
The gate bursts open, and a mass of black engulfs the place.
The heroic spirit Emiya had left his past self behind, returning back to the Throne of Heroes to await his next summoning to perform his duty as a Counter Guardian. However, en route, something else happened, and he was shunted off towards another destination entirely.
I don't know what's going on... but I certainly blame Zelretch for this! Despite his unawareness of the circumstances involved, Emiya managed to lay the blame at the culprit through past experience.
Emiya found himself... in the world cast by his Reality Marble; a smoky desert plain littered with an infinite number of swords, while giant gears shifted in the sky above. Except there was a certain difference...
"Hello, are you a hero? My name is Harry Potter." A young boy with vivid green eyes greeted him nervously on the same plain.
Curse the Root. Emiya wondered just what had he gotten himself into.
Not really a good way to end it, but meh.
Okay, to sum things up, this is mainly Harry Potter, except he has somehow (read: will of the writer) performed a summoning ritual cum alchemy exchange due to Zelretch's meddling? Interference? Well, whatever it is, we have Archer summoned into the body of young Harry Potter, with the toll paid to the Gate (from Fullmetal Alchemist) due to the destruction of the Holy Grail at the end of the UBW route. Realistically, Harry shouldn't be able to memorize the full chant (I modified mine from the one in the UBW movie) or draw a proper summoning circle, so I simply twisted things such that a one in a billion occurence actually occurs. It's pushing the bounds of reasonability quite a bit, but heck, I just want to get this out of my head.
Anyway, my intention is that Harry Potter's magic takes the most precedence here, but he gets to perform Archer's Nasuverse magic due to the existence of Archer inside him, thus Harry's 'power that the dark lord knows not' includes Tracing and Reinforcement, but not the Reality Marble as Harry's soul is not the same as Archer's, and thus he cannot project Archer's Reality Marble. Not unless they both are in tune with the same ideals.
I will add another disclaimer that states that I am not very experienced with Nasuverse rules, and am just winging it according to the fanfics I've read.
Not sure whether I'll continue this, but let's just see how.