My most popular Harry Potter crossover seems to be Haemophilia, which was derived from the only three chapters published (as of publishing this chapter, anyway) of It's in the Blood, by sakurademonalchemist, one of my favourite authors on this website. It is thanks to her generosity that I am adapting another of her stories, or at least the basic concept of it.

Her story Gorgon involved what was presumably an iteration of Illya from Fate/Stay Night: Unlimited Blade Works tumbling back in time after being killed by Gilgamesh. She ends up in an eleven year old body in Diagon Alley at the same time as Harry Potter does, and Illya ends up going to Hogwarts with Harry, where they're both Sorted into Slytherin. Sadly, Gorgon never made it past seven chapters. But the idea of Illya being shipped with Harry was an intriguing one, though it needed tinkering with for blatantly obvious reasons: to whit, getting rid of her childish appearance.

With sakurademonalchemist's permission, I have written my own version of Gorgon, where an UBW Illya is sent back in time in a body her actual age (18 or 19 instead of a kid's body, which bypasses the main problem with shipping Illya with, well, just about everyone and anyone) to try and stop the events of Fate/Stay Night from happening, and ends up with a post-Hogwarts Harry. I originally had Illya have her memories intact, but the story got bogged down in too much exposition, and didn't get to the Harry/Illya quickly enough. I thought it might be more interesting to have Illya lose most of her memories thanks to Angra Mainyu being a dick, and have her regain them in time for she and Harry to get involved in the events of Fate/Zero. And Harry would be forced to confront the possibility that what he knew about his wife was wrong. Oh, and Illya will actually get to see her father in action as the Magus Killer.

Res Nullius (which means 'nobody's property', and is meant to refer to Harry and Illya shaking off their manipulators) may have taken Gorgon as a basis, but it is, hopefully, its own beast, and I hope you enjoy the sample chapters I post. I will also be posting the first four chapters of Underground's Champion and the first NINE chapters (!) of Gorgon and Thanatos at about the same time as this, so keep an eye out for them...

EDIT (2/1/2017): Res Nullius, due to some consideration, may not see the light of day, at least as a Harry Potter crossover, as I think Harry is superfluous to the story. I'm strongly considering adapting it into a pure Fate/Zero Peggy Sue story with the same premise, pairing an adult Illya with either Waver or Kariya, or even Kirei (yes, I am considering that, believe it or not). I will be posting the existing chapters already written, for the interest of those who want to see what I've written so far: three chapters including the prologue.

RES NULLIUS

PROLOGUE:

TIME WARP

She hadn't expected to wake after the blonde-haired Servant with the blood-coloured eyes literally tore her heart from her chest. She had expected oblivion. Instead, she woke within the Grail, on a dark beach.

She only remembered her time in there but dimly, but her mother was there, with the truth. Her mother, the last vestige of her mother's soul that wasn't corrupted by Angra Mainyu, the Avenger Servant her own damn family had summoned to cheat during the Third Holy Grail War. She held her, comforted her, told her what had really happened, how the von Einzberns deliberately kept her father away from her, and then shaped his daughter into a killing machine to murder a boy she was told her father had abandoned her for.

She had cried, and cried, while her mother soothed her. Then, her mother outlined an audacious, insane plan. The core of the Grail still worked. With the right boost, it could fulfil its purpose without unleashing Angra Mainyu on all creation. And with that boost, she would have a second chance, even a new body that wasn't stuck so young.

When the arrogant Gilgamesh, Archer of the previous Grail War, was sucked into the Grail, they knew the time had come. The sheer raw power of that Servant would work well. Plus, the irony was appealing, to use her own murderer to give herself a second chance.

Her mother was going to send her back, to where she was needed. She was going to find the best protector for her, as well as someone who could be her friend, give her the happiness she had been yearning for all her life. She hoped so. She fervently hoped so.

As the landscape around them turned from darkest night to brightest day, the daughter looked at her mother. "Goodbye, Mummy. I'll see you as soon as I can."

As everything dissolved into white light, her mother said, "Good luck, my daughter…"

But even as she left, she felt something reach into her. Angra Mainyu, unable to stop her, was trying to tear away as much as it could. She felt her memory being shredded to ribbons, not actually gone, but tattered and left in useless ribbons, trailing in her wake as she hurtled back. To the beginning. And beyond…

It wasn't exactly the worst day of Harry Potter's life, though it was tentatively making it into the Top 10. It was certainly in the Top 3 of worst days that didn't happen on Halloween (automatically excluding the night of his parents' deaths, as well as many of his Hogwarts worst days). However you sliced it, Harry was in a foul mood.

Vernon Dursley had died. Predictably, it was of a heart attack. Harry felt it was good riddance to bad rubbish, but went to the funeral anyway. Unfortunately, his aunt had verbally attacked him, calling him a freak (ironically, it was Dudley, of all people, who tried to calm his mother down). And then, the fucking wizarding paparazzi had to show up.

Not content with intruding on Harry's life once more, during a time that was meant to be private (though Harry was secretly hoping for enough alone time to piss all over Vernon's grave), they began asking questions as to which eligible witch he would be marrying. He told them to piss off. And then, one of them claimed that Ginny Weasley had announced her betrothal, which was news to him, as he had only just started going out with her on a few tentative dates. He had reacted badly to that, saying he hadn't consented, and if Ginny had stated that, then it was over.

Sadly, she had, and she and Molly wouldn't take no for an answer. Ron seemed to just stay out of it: with Hermione dating Neville (of all people!), he had decided to stay out of the dating game, and he wasn't surprised that Harry wanted to too. Harry had managed to declare the betrothal null and void, despite Ginny and Molly's protests, and had decided to shut himself away in 12 Grimmauld Place. There, he had availed himself of his godfather's rather large collection of spirits and liquors, and was trying to drink himself into a stupor. As it was, he vomited up too much to get drunk beyond 'mildly plastered', and he resigned himself to less of a buzz than before. To try and cheer himself up, he decided to raid Sirius' pornography collection, hidden in the Black family library, and thankfully not removed by Molly Weasley (more by accident than design, he guessed).

So when a naked albino girl about his age seemed to fall out of the middle of the air in the library, he took a moment to look at the glass still in his hand (the rather raunchy book in the other), wondering whether he was hallucinating. After a moment, he decided that he hadn't drunk enough to hallucinate anything, let alone albino girls about his age somehow Apparating into this house.

"Kreacher?" he asked, tentatively.

The ornery, ancient House Elf appeared. "Yes, Master?" he asked. He had gotten better disposed towards Harry since destroying the Locket that had gotten Regulus Black killed.

"We appear to have an unexpected guest. Could you please bring her to one of the bedrooms…and find some clothes in her size?"

Kreacher nodded, before he sniffed the girl. "Half-breed," he hissed.

"Kreacher, what have I told you?" Harry said warningly.

"Master, she is a half-breed, not even a witch. She is half-Magus, half-Homunculus."

Harry blinked. Magus? He sort of vaguely knew what a Homunculus was, but a Magus? "Just do what I told you, please, Kreacher."

Consciousness returned slowly, gradually. She thought she heard English, rather muzzily. But she slept on, dreaming. Of a boy with red hair and golden eyes. Of a dark-haired girl with a red dress. Of a blonde girl with green eyes and a regal demeanour, wearing a blue, armoured dress. Of a tall, tanned man with white hair, wearing a red coat over black armour. Of her two maids, almost identical in appearance, but not in demeanour. Of a massive, musclebound giant with skin that made him look like he was a statue, hewn from dark stone. Of her mother, those features so much like her own, and yet an adult body. And of her father, dark-haired and weary, with a sadness in his eyes.

Eventually, she woke, slowly and reluctantly. Her first realisation was that she was in bed, and utterly naked. A quick spell showed that nothing untoward had been done to her. But her body seemed strange now. So did her mind. So full of holes.

As she managed to get out of the bed, she found herself facing a body-length mirror, and she stared in shock. Okay, she should have realised the moment her limbs seemed longer than they had been, or that her chest was much heavier than she was used to…and yet, she was still surprised. She stared at the body of a stranger…

No, not a stranger. It was her. She had managed to reach her true age, somehow. She was eighteen, but stuck in the body of a pre-teen. She didn't remember much, but she could remember that much. Now, she actually looked eighteen, and a very good eighteen at that. She had proper breasts now, for crying out loud! And those legs! Long, graceful legs! A small chuckle wormed its way from her lips. A last gift from her mother, she thought. An actual body that wasn't a prison.

She twirled in front of the mirror, giggling quietly to herself. She could still feel her Magic Circuits, too. She was still as powerful as she was before, if not more so. The rush of power was intoxicating.

She brought herself into calm with difficulty. She was still naked in an unknown situation. Pursing her lips, she studied the room, reaching out with senses honed by her abilities as both a Magus and…something else. She couldn't remember what that was for the life of her. The magic ambience already told her one thing: she wasn't in a Magus household. She was in a wizarding one.

That could be good or bad. Wizards and witches were the old-fashioned 'cousins' of Magi. They had a Magical Core rather than Magic Circuits, which meant they were far less at risk of incurring the wrath of Gaia, and they were less ruthless, but on the other hand, many were even more contemptuous towards non-magicals than Magi, due to Blood Purity issues that were a corrupted version of Magus doctrine (that of longer-established Magi lines having a better connection to magic). Not to mention that they were backwards and conservative, even compared to Magi, their magic was less versatile and required foci more often than Magecraft did, and they actually believed in prophecy. She didn't know all the differences, and she could care less.

The décor was pretty dark, and while she couldn't be certain, she was guessing she was in England. There was a portrait (with the occupant currently missing, thankfully) of someone called 'Cassiopeia Black', and the Black family was one of the more notorious in Magical Britain.

Still, no spells keeping her in the room, and there was clothing folded up and placed on a nearby chair. A knee-length skirt, a blouse, and undergarments. She opted to get dressed once she had finished her morning ablutions, which she did, savouring a nice, long bath.

Getting dressed, she opened the door, and found a faint smell of cooking wafting into her nose. She snuck downstairs, and carefully padded her way through the rather dark and dreary house, trying to find its origin. It was hunger and curiosity both that led her to the kitchen.

As she opened the door, she soon saw her host, and stared. Of all the people to be cooking breakfast for her, it had to be the Boy Who Lived. About her age, with that infamous scar fading. Scrawny, but handsome, with messy black hair framing his face, emerald eyes flashing behind glasses as he cooked.

With that, she cleared her throat to get his attention.

Harry suppressed his surprise when he heard her clear her throat, instead, saying, "Breakfast is almost ready. I had to persuade Kreacher to let me do it. You want some?"

The girl, after a moment, smiled. "Yes, I would, actually. Thank you," she said, in a voice that actually had more than a touch of a German accent.

Harry nodded, and began serving it up, as it was finished. As he did so, he took the time to properly scrutinise his guest. Given his inebriated state last night, he hadn't managed to do so before (plus, he still had to take a bloody hangover potion). As it was, he wished he had been sober last night.

The girl was surprisingly tall, about as tall as he was (and he had been rather short for his age, thanks in part to the Dursleys). Her skin was so pale as to be white, and her hair was like snow and ice spun into hair. Her features were inhumanly beautiful, and he had to wonder briefly whether she was part-Veela. Her eyes had a slight epicanthic fold, suggesting Asian heritage, though her irises, strangely enough, were blood red. Her figure was attractive too.

She took the plate gratefully, and began to eat with a surprising dignity and grace. They didn't speak over breakfast, instead committing merely to eating. After she had finished, she said, with a smile, "Thank you. It is rather plebeian food, but well-cooked."

"Plebeian?" Harry asked, annoyed. "Are you a Pureblood?"

She blinked. "I…I'm not a wizard. I'm a Magus. But I cannot remember where I grew up. I'm sorry. And that's not saying your food is bad. Far from it," the girl said. She pouted in a manner that made her almost endearingly childish. "Damnation, this is hard. I don't know how I got here, I'm missing a lot of my memories, and I'm tripping over myself. I didn't intend to offend you, Harry Potter. Yes, I know who you are. The scar's pretty distinctive."

Harry frowned. She was claiming to have amnesia? Something about her made him a little wary, understandably. But after a moment, he asked, "Well, do you remember your name?"

The girl blinked again, and frowned. Eventually, she said, "Yes, I do." A smile lit up her face. "I remember my name!" Her face fell. "Well, my first name. And I can't remember my parents, though. But I am Illyasviel. Call me Illya. Pleased to meet you, Harry Potter…"

PROLOGUE ANNOTATIONS:

So, here you have it. Illya's ended up with a post-Hogwarts Harry. And in case you're wondering, Irisviel (as part of the Grail that isn't corrupted by Angra Mainyu) has sent her back with a body that is her actual age, and back shortly before the events of Fate/Zero. For the purpose of this fanfic, the events of Fate/Zero take place in early 2001 (rather than the generally accepted 1994), with this chapter taking place in mid-1999. That means plenty of time for some Harry/Illya before the Grail War kicks off.

I thought of Illya as looking like a version of her mother in her late teens. I mentioned epicanthic folds due to her father being the Japanese Kiritsugu Emiya, and Iri does mention in the opening episode of Fate/Zero that Illya has his eyes.

Now, originally, I intended to have Illya come back in time with her memories intact, but I thought that'd create too many problems, having her know too much about the future. So I decided to have Angra Mainyu be a dick and take a good chunk of her memories. She knows her name, her magecraft, and about the Grail War, but she's forgotten her family, along with the events of Fate/Stay Night. She'll recover them in time for the events of Fate/Zero.

There's another reason for that. Because she retained her memories, the chapters I initially wrote were filled with exposition, and it would have taken forever to get to the Harry/Illya romance. I'm going to skip forward to shortly before the events of Fate/Zero, with all the lead-up to the romance taking place offscreen, and Harry and Illya already being spouses and lovers. And with her regaining her memories…

No numbered annotations this time.