Thanks for the overwhelming support for Res Nullius, but I have some bad news...and some good news. The bad news is, I realised that this was a pretty weak crossover with Harry Potter, and could work well without the Potterverse elements. The good news is that it can work very well as a pure Fate/Zero story, as a Peggy Sue/time travel fanfic. Instead of Harry, I'm considering one of three pairings for an adult Illya: Waver, Kariya, and Kirei Kotomine (yes, I am seriously considering that: I think the Kirei of Fate/Zero can be steered away from becoming his Fate/Stay Night incarnation). It's still in its early stages. As I have written three chapters of Res Nullius already, I thought I would post what I have written here already for your reading pleasure, as well as give a taste of what might be in store when I get around to writing the actual fic. Hope you enjoy it, and hope you guys aren't too disappointed.

RES NULLIUS

CHAPTER 1:

A LIFE REGAINED

Over a year and a half later…

She's so cute when asleep, Harry thought as he woke up to find his wife curled up next to him, as naked as he was. He gently ran his hand through her hair, causing her to giggle softly. "Harry…" she murmured, half-asleep. She opened her eyes, and propped herself up on her elbows, looking at him with those crimson eyes of hers, smiling that gorgeous smile. "Good morning."

"Morning," Harry said. "Sleep well?"

"I always sleep well with you. Especially when we have a good time the night before," Illya said, her smile becoming a somewhat lascivious smirk. "Kreacher's probably glad we put up those extra privacy charms."

"I must be doing a good job if you're screaming that loudly in pleasure, Illya," Harry said, matching her smirk.

She leaned forward, and kissed him, a brief peck on the lips. "That you do, Harry." She began stroking his member until it was standing to attention, before shuffling over and straddling him, allowing him to enter her, Harry casting a contraceptive charm as she did so. "Let's work up an appetite for breakfast, shall we?" she asked, a wry smile on her face.

Over a year and a half had passed since Illya fell almost literally into his life. A mild distrust of the young woman soon gave way to friendship, and sometime after that, it developed into love. It was an uneasy love: even if it wasn't suspicious, which it was, Harry knew that he could be loving a young woman who may already have a husband, or at least a boyfriend. And yet, despite the unease, it had developed into something strong and undeniable, even as they both tried to piece together her past, with limited success.

Illyasviel, they had surmised, had probably been born of a human and a Homunculus, in all likelihood, the von Einzbern family, a reclusive but powerful Magus family who resided in a remote corner of the Black Forest of southwestern Germany. They specialised in alchemy and the creation of Homunculi, the latter of which often had the distinctive albino-like look.

Illya, upon learning this, had asked Harry that they not attempt to make contact. Harry agreed, as she seemed worried about what they'd do to her, which could be vivisection at the very least. He had learned that Magi were notoriously amoral as a whole, and at best.

Partly to protect her from the von Einzberns, and partly to prevent any betrothals to Harry from getting anywhere (especially from a desperate Ginny and her mother), Harry and Illya married a few months ago. Harry and Illya would have been happy to remain lovers, but the pragmatism of the protections offered by marriage helped solidify their decision.

Harry, through his wife, learned about magecraft. He could only learn the theory, as well as what few spells that could be learned by both Magi and wizards. She seemed to retain her ability to use magecraft, and knew a little of its history.

Of course, he learned as much through his new position. In order to distance Harry from the scandal his continual rejection of the Weasley betrothals (amongst others) were causing, Kingsley had suggested Harry for a post that many in the Ministry saw as a joke. The Head of the Magus Liaison Department, effectively meaning he had an office in Clock Tower.

That was actually a surprisingly interesting job, and not just interesting in the curse sense of the word, though it certainly was that. Clock Tower was rife with intrigue and politics that was the equal of the Ministry of Magic. Thankfully, Harry had managed to get on the right side of a few of the key members of Clock Tower, most important of whom was Kischur Zelretch Schweinorg, or Zelretch for short. A tall, powerfully built man apparently in his seventies, he was actually a vampire, or rather, a Dead Apostle Ancestor, a specific type of vampire. While not evil, Zelretch was an inveterate prankster who had, on a couple of occasions, had corresponded with the Marauders after they left Hogwarts. He was also one of George Weasley's biggest customers. And he was Luna Lovegood's honorary grandfather, of all people, which showed how small a world it really was.

Through him, he met the woman who was Zelretch's honorary granddaughter, and Luna's honorary sister, Arcueid Brunested, aka Arc, the White Princess of the True Ancestors, a somewhat different breed of vampire. While not actually affiliated with the Magus Association that ran Clock Tower, being able to claim a friendship with her and Zelretch kept a lot of people off his back. The fact that Arc was a good-natured person helped matters.

Not that he didn't make enemies, though. His cheerily iconoclastic attitude made him a few. There was one rather nasty instructor who was basically a more competent and intelligent Malfoy that got Harry's ire up. He had the improbable name (though not as improbable as some wizards he knew) of Kayneth El-Melloi Archibald, an elitist ponce who unfortunately could back up his haughtiness with actual ability. That didn't stop Harry from taking him down a few pegs, as well as encouraging one of his students, Waver Velvet, in writing a thesis about how hard work can allow a Magus to become more powerful than one from an older family with a longer connection to magic. Okay, the Magi actually had an intelligent reason to discriminate against those with a more recent Magus lineage, as the longer a family existed, the more powerful the connection to magic (of the magecraft variety, anyway), but that didn't stop Harry from taking every opportunity he could to annoy them. He hated that elitist attitude.

While Harry worked at Clock Tower, Illya opted to work at Diagon Alley, at Flourish and Blott's. Not that either of them needed the money, what with his family money to fall back on, but neither liked to be idle.

Of those he knew, the only ones who had actually met Illya were Hermione, Neville, Ron and Luna. He didn't really trust anyone else. Ron had the unenviable task of relaying to his mother and sister that Harry was married now.

All in all, it was a good life. Not quite quiet, but certainly a good life. Unfortunately, today was the day it would change…

After their little morning workout, Harry and Illya showered, dressed in silk dressing gowns, and went down to have breakfast. It was Saturday, and Harry intended to take his wife out for a day trip around London. She enjoyed sightseeing and going to the various attractions of London. Yet Harry noticed a strange melancholy, even an anticipation. As if Illya knew, on some level, that something was soon to happen.

Harry began reading through the morning mail, brought by Owl Post. Zelretch sometimes used the wizarding method of messaging, simply because the conceit of using owls as messenger birds amused him. There was a lengthy letter from him now.

As he read down the letter, Harry scoffed quietly. "What is it?" Illya asked.

"Zelretch wrote to me. Apparently something called the Holy Grail War is starting up in Japan before long. He thinks he can get me a spot, as the Magus Association is only sending in that wanker Kayneth. He thinks it'd be interesting to have a wizard as a Master, whatever that means. He says I can see him tomorrow to learn further details." A strangled noise from his wife brought his attention away from the letter rather hurriedly. He realised that Illya was quaking and quivering. "Illya? What's wrong?"

"I…Grail…remem…ber…" Her eyes rolled up in the back of her head, and she toppled out of her chair to the floor, convulsing and writhing, as if in the grip of a massive seizure. She began screaming in pain, her voice rapidly hoarsening. Harry was by her side in a flash, holding her, hoping to stop her from hurting herself.

As if by his touch, she calmed, subsiding, her eyes, instead of rolled up, were now staring straight ahead, fixed into the distance, twitching rapidly. What was she seeing?

Illya remembered.

She nearly drowned in a deluge of memory, memory that had been nearly torn away by Angra Mainyu. It wasn't until she felt Harry's touch, the feel of his magic, that she was able to stay afloat in the flood. Part of her managed to calm down. The rest of her still felt all those memories flooding back.

She could have been left a vegetable, but Illyasviel von Einzbern was stronger than that. She had controlled Heracles as a Berserker! She was the daughter of the Magus Killer! And the wife of the Boy Who Lived!

It was hard keeping her head above water, to use a metaphor, but she managed. It was an effort, but she managed. After what seemed like an eternity, the flow of memories slowed, and then ceased. It was still a chaotic swirl, but she now knew she had everything. What's more, she knew that something had to be done about it.

She came back to herself to find her husband staring at her in fear and concern. His words finally came to her, as if through a mist. "…ya?! Illya! Are you all right?"

"Harry," she said quietly, reaching out a hand to touch his face.

"What's the matter, Illya?" he asked. Oh, sweet Harry. He could tell that something was bothering her.

The next three words to come from her lips were amongst the hardest she had ever spoken, surpassed only when she first told Harry "I love you". But speak them, she had to. So much was at stake.

And so, she spoke them.

"I remember everything."

An hour later, Harry emerged from the Black family Pensieve, staring at his wife in horror. She had offered proof of what she knew via copies of her memories, and thankfully, the Black family had their own Pensieve. Illyasviel's story was a tangled one, as much as his own life had been. Illya was born to Irisviel von Einzbern, a Homunculus created by the von Einzbern family, and Kiritsugu Emiya, a Magus of some infamy as a mercenary and assassin, known as the Magus Killer. Thanks to experiments on her while she was in the womb, as well as when she was a child, her physical development was slow, and ground to a halt before she reached puberty. Until now, she had been stuck in a child's body, despite having lived to the age of 18.

Anyway, her father had been retained as the chosen Master representing the von Einzbern family in the Fourth Holy Grail War, a magical tournament that made the Tri-Wizard Tournament look like the Ashes(1) by comparison. Seven Magi would summon the spirits of heroes from myth and history (dubbed 'Servants') to do battle, each in a specific class, in the Japanese city of Fuyuki. Her father had been the Master of Saber in that Grail War, but had never come back from Fuyuki, having destroyed the Grail and betrayed the von Einzberns.

Illya's 'grandfather', Jubstacheit, along with a dream vision of Illya's mother, had claimed Kiritsugu had abandoned Illya, in favour of adopting a Japanese boy called Shirou. In truth, Illya would later learn from the soul of her true mother within the Grail that Kiritsugu had tried to retrieve Illya, only to be rebuffed by the von Einzberns. And the von Einzberns not only expected Illya to be a Master in the next Grail War, but also become the next Lesser Grail, a vessel designed to become the next Greater Grail, the Holy Grail proper, when charged with the lifeforce of at least six Servants.

Oh, and they wanted her to murder her stepbrother, as her father had by then died from a curse.

Illya became the Master of Berserker, who was actually Heracles. Heading to Fuyuki, she had a number of battles, including with the very Saber Servant her father summoned, and who was now protecting her stepbrother. But during the Grail War, she was attacked by a mysterious Servant with golden hair and red eyes, whom she later identified as Gilgamesh. He tore her heart from her after defeating Berserker, and used it for his own version of the Grail.

It was there that Illya met her mother for the first time in years. Irisviel was still part of the Grail, and part of her had remained untouched by the entity corrupting it, Angra Mainyu, supposedly the devil-figure from ancient Zoroastrianism. And through Irisviel, they had enough power to send Illya's soul, along with a new body (closer to her actual age) back in time.

Harry knew now, given that he had seen her memories, that his wife was a dangerous young woman, more so than he had before. But he had also seen her heartbreak and her hurt. And she was shaped into a sacrificial weapon by an ancient old man, like he was. Only, Dumbledore had at least good intentions, despite what the road to hell being paved with. At least Dumbledore wanted Voldemort dead, and had tried to find ways for Harry to survive the experience. Jubstacheit, however, just wanted the Holy Grail, despite him sacrificing the happiness of a young girl who only wanted to see her parents again.

Illya, after a moment, said, "I'm sorry. I mean, if you hate me, I…"

"I can't hate you, Illya. I'm a little angry at you for wanting to murder Shirou based on Acht's say-so, but…you know what you did was wrong now. Anyway, you showed me your memories the moment you regained them." Harry gave a sardonic grin. "And Dumbledore had a thing for giving second chances."

Illya laughed sadly, tears running down her face, before throwing her arms around Harry and embracing him. "Harry…thank you," she said quietly. "You're the first, you know. The first boy I ever loved. Had it been in my original time, I would not have even given a wizard a second glance. But you've been so good to me."

"It's kind of hard to get a boyfriend when you're stuck in an underage body," Harry remarked. "I'm glad you look your age now."

"Of course you'd be," Illya said with a cheeky grin. But then, her face became serious. "Harry…I need to ask you…I need your help. You know what lurks within the Grail. Angra Mainyu, the Avenger Servant my family tried to summon. I need to find a way to destroy the Grail, or else dismantle it so that it isn't in danger of contaminating Akasha, not to mention destroying humanity. I don't think contacting my family is a good idea: they'd either kill me for being an impostor, or vivisect me. I might be able to persuade my parents once we're in Fuyuki. Zelretch may believe me, but…"

"You want me to go with you, fight in this Grail War, against a group of ruthless Magi, including your own father?" Harry asked. After a moment, he muttered, "Too much to ask for a quiet life, isn't it? Okay, I'm in. Two's better than one, and if we're summoning Servants, then four's better than two, right?"

Illya smiled. "I'm glad you see it that way, Harry. Now, I think we should change the destination of our daytrip. I think we should take a little trip to Cornwall…"

CHAPTER 1 ANNOTATIONS:

So, Illya remembers, Harry knows the truth, and it seems that Illya has a plan. For those of you familiar with Fate/Zero, Illya's plan is somewhat obvious. For those of you who are not…well, wait and see.

1. For the Americans, the Ashes is a test cricket series between England and Australia.