Disclaimer: I have no claim to ownership of either of the Harry Potter or Nasuverse series. HP belongs to JK Rowling, the publishers, movies studios etc. The Nasuverse belongs to Type Moon etc.

A/N: This FF is, obviously, somewhat AU. Though, obviously, expect spoilers. HP Timeline is untouched, FSN's has been shunted to the modern day (201X rather than 200X).

HP will be treated as (mostly) per Canon up until after the summer after the Battle at the DoM, with most of HBP and DH still taking place, with a few minor tweaks, such as Harry having a big ol' trunk'o'moniez, (and several other compartments besides), Harry's blood having been absorbed by Gyffindor's sword, in addition to the basilisk venom (giant snake fang in the arm is not going to be bloodless), and Harry being an Animagus (ever since sixth year), with said ability partially bound by the grail (as the grail is a thaumaturgic binding object, and the creatures in question are classed as 'Demonic' beings in the nasuverse).

This FF is an alternate for the Fifth Grail War. (Fate/Stay Night), and I'll be drawing on elements from the Fate, Unlimited Bladeworks, and Heaven's Feel (just a bit) paths. This is not the canon fifth grail war.

That being said, this is NOT going to follow the path of HF. No Dark Grail!Sakura and corrupted servants.

Harry is taking the place of Medea, and will not be summoning servant assassin for himself. Assassin will be already summoned, and by a different master. I haven't decided whether to use the False!Assassin summoned by Medea or another. (Maybe a True!Assassin [of the Hassan-i-Sabah], maybe a different game's assassin, maybe an OC or char from another game altogether.)

Pairings will be Shirou/Rin/Sakura and Harry/Illyasviel, nothing explicit. Assume Everything as canon unless extrapolated upon otherwise.

[i]You would be shocked at how little difference there is between most alternate universes.

Now, what you must understand, is that any given universe is the sum of an infinite number of things happening or not happening in combination, and any single change, to any single one of this infinite number of events is it's own alternate dimension, even should every other event occur in exactly the same way. Quite often the differences between universes can be as simple as someone, not even anyone important, choosing to have a ham sandwich instead of a chicken one. And since there are an infinite number of events comprising any given universe, there are therefore an infinite number of alternate universes that vary only infinitesimally.

But likewise, there is an infinite subset of universes with larger variations. Where things that never existed in one world exist in another. Like magecraft. Like magic. Like where crabs developed to become the dominant lifeform. And some of these worlds can be strange indeed.

And there is a smaller, yet still infinite subset where these sometimes happen to intersect. And even these intersections can occur in an infinite number of ways.

And an even smaller, yet equally infinite subset in which events are recorded.

This is one such story.

-An Excerpt from the living memoirs of Zelretch, in the Tohsaka family library. If you found parts of it confusing, that's probably for the best.[/i]

And events were unfolding. Just as they always had, just as they would on an infinite number of other worlds almost exactly like this one, and an equally infinite number a bit more different.

May 3rd, 2011, a home in Fuyuki City, Japan.

With a obscuring billow of bright green mist, a figure stood. He, for the voice was definitely male, groaned. "No Sword of Gryffindor, so I'm not Saber, haven't got my Cloak on hand, so not Assassin, not that I was all that stealthy really, more a Gryffindors charge ahead kinda person, never been a Lancer, too in control of myself to be 'Zerker, and I never went insane in life, I doubt that I'd count as Archer, even though I work at range. Question is whether I'm here for my days of Thestrals, Hippogryphs and Broomsticks, that dragon doesn't really count, since I was just a passenger, though I'm a bit short on Phantasms... Or Magic..." The figure began to pat himself down. "Holly-Phoenix, repaired too, check. Hawthorn-Unicorn? Huh, Draco's wand. Yew-Phoenix? How did I end up with Riddle's? Ahh, Elder-Thestral. Feels right. Guess I'm here as Caster."

The mist cleared, revealing a black-robed man with unruly raven-hued hair. "You are the Servant Caster?" The voice was snappish. As though it somehow doubted itself. Or maybe what it saw.

The young man - no, the summoned servant - looked up. His piercing green eyes, shimmering with raw power, was it's own answer. "You are the one who summoned me." It wasn't a question. "I, servant Caster, have come forth in response to your summons. What are your orders, Master." The binding words spoken, and on the master's hand, a trio of stylised red-orange markings were etched into the skin. A line, within a circle, within a triangle. "From this time forth, my wand shall be with you and your fate will be with me. - Now, our contact is complete." The dark-haired, green-eyed servent stretched, rotating his joints until they 'popped', then eyed his master up. To be honest, Caster didn't like what he saw. His newest master was a greasy young man, barely sixteen, if Caster was any judge, sallow skinned, with a hooked nose. "Woah. Deja vu. Back when I was... Alive, for lack of a better word, I knew someone that looked a lot like you. Nasty bugger. Brave as hell, in the end, but still a belligerent arsehole. Prove that he was an exception, rather than the standard. I'm assuming you understand the basics of the command spells and how to identify a servant's status?"

The sallow one nodded. "The command spells force the servant to act according to an order. It stops unwanted actions through pain responses and strengthens desired ones."

The servant bobbed his head, ever so slightly. "The more specific an order, the stronger the effect. Three crystallisations of the Grail's own magic that allows the servant to surpass their limits. They work poorly on broad and ill-defined orders. An order to 'win this battle' isn't going to do much. The pain responses or power enhancements are diminished. To contrast, a simple order for a single act, such as 'stop your attack' is absolute, and near impossible for a servant to do otherwise. I have a couple questions of my own. What year is it?"

"2011."

He flicked his hand in a peculiar pattern, waving around a stick, and a date appeared, formed out of smoke. "Thirteen years since I died, plus a week. Ten years after the fourth grail war, which was only three years after I died. I wasn't expecting another grail war to break out for another fifty years though."

"You were in the previous war of the holy grail?"

"I was. My master was a bloody incompetent with very little skill of his own, and got himself killed thanks to Servant Assassin finding him as he went to the pub. I assume you're not going to make that mistake?"

"I- I'm too young to drink."

"Like that ever stopped anyone. It's a shame that I ended up bound to you, young magus. The prana drain incurred on the master as an active servant caster is higher than any barring the master controlling a servant berserker. But you only have a middling amount of magical strength, thus I fear you will lack in power to protect yourself. That, and I will be unable to utilise my greatest Noble Phantasm."

The sallow-skinned master frowned. "And what is your true name, Servant Caster?"

"Harry Potter."

The sallow one frowned again. "I don't recognise the name. Are you a mere low ranked spirit? Your abilities? Noble Phantasm?"

"I'm a wizard of the highest caliber. Once I reached my magical maturity, with it's full magical core expansion, and the bindings on it severed, only one living - being - had the power to match me. As for my Phantasms, of which I only seem to have one third of a set, one of the two physical object phantasms that should have been summoned with me, probably on account of your insufficient power, there are three. The first is a blade, with which I slew a King of Serpents in life, as well as shattering some objects of darkest magic. The second is a trio of items, crafted by none other than Death itself. A wand of immense power, useless to your magics of course, optimal for mine, but crafted with some of the magics of Death's own steeds. A stone that allows for the ressurection of souls, or bodies without them. And a cloak that renders the wearer invisible and undetectable." He frowned, briefly thinking of the times that had proved to be less than accurate, "As for the third, it's an inherent magic, which the Grail itself has seen fit to bind to the Servant bond, which means I'd be forced to draw upon your prana rather than just my own."

"And what is this mysterious inherent magic?"

"A self-transformation, in which I take the form of an immense, magically powerful beast, while retaining my own magics and intellegence. But it matters not, since I can't use it anyway."

Within her mansion, a young silver-haired, crimson-eyed homunculus, Illyasviel von Einzbern, or simply 'Ilya' to her friends, had just heard notification of the successful summoning of one of the seven servants of the holy grail, this time a barking-screech of a snowy owl, oddly crossed with the trill of a songbird. Her own servant, the first summoned, was a fourteen foot tall, muscle bound, bronzed mass of raw hatred. The Demigod Heracles. Servant Berserker. The servant with the noble phantasm God Hand. Gifted with twelve lives, to match the twelve tasks he completed in life.

Why the bark of a snowy owl? It was all to do with the used catalyst. Her own Berserker was summoned using the very same slab of rock he now carried as a blade, origially culled from a temple to Heracles himself. The blue lancer had been summoned with aid of the very same jewel he'd been entombed with on his death. As for the songbird, Ilya didn't even know which species it was, so it could take years, decades even, just to find the relevant infomation.

Five of the seven servants of the Holy Grail had now been summoned. The fifth war of the holy grail would begin within the week.

Sometimes, it's the smallest things that can divert the course of any alternate universe. Some changes can be anything but small, yet still change very little of the greater scheme of things.

Some changes can divert the path of a universe so radically that it becomes almost unrecognisable from it's 'peers'.

The question is truly to work out just which changes make the big differences.