I'm crossposting this in both The Cauldron and Zelretch's Collection of Alternates.

Now, this is a story that, frankly, has been quite a while in the making. It's a Harry/Arturia story that I've wanted to do for a long time. Now, I know what you're saying, I already have Ex Umbra in Solem floating around. And that is true. But I've been struggling with that story for a while now, and while it's by no means abandoned, it is, more or less, on hiatus until I can get more inspiration for it.

This isn't my first attempt at another Harry/Arturia story either. Followers of The Cauldron and Zelretch's Collection of Alternates will remember Non Omnis Moriar, inspired by sakurademonalchemist's abortive similar story Rebirth of Phoenix. I'm still considering doing a similar story, but this story came out first.

It's a combination of a number of ideas. Aside from Rebirth of Phoenix and Non Omnis Moriar, there's also a story concept taken from DaSalvatore's excellent story Rebirth of the Founders, a story which I highly recommend. Plus, some elements of the story echo my previous story Gorgon and Thanatos, as well as my Final Fantasy IX crossover Kuja von Einzbern. I thought it would be fresh because Harry is not aligned with Kiritsugu Emiya, and thus has a different perspective on the Grail War, tempered by his physical youth. Anyway, I hope you enjoy...

EDIT (19/11/18): Yeah, I know, this is probably getting tedious...but after some thought and discussion, I've realised, I don't quite want to do yet another Fate/Zero crossover, not in this way, and the plot after a certain point wasn't working out. So...well, remember how I said this was based on one of Dis Lexic's challenges? Well, the new version will be an actual answer to it. Watch this space...

KING OF HIS HEART (ORIGINAL)

CHAPTER 1:

YET ANOTHER FINE MESS…

"You bastard. You effeminate cambion bastard. Did you know about this? I'll bet you knew something."

The man with the long face framed by dark hair knelt down next to the body sitting up against the tree. The fools deluded themselves into believing her to be a man, stuck as an eternal youth. But the man knew better. He stroked the cheek of the woman he loved, the woman who chose to become a king because of her bloodline. She could have been sleeping, but he knew better.

"Ria…" the man said quietly, tears trickling down from his emerald eyes. "I'm sorry…I should have been here. But Godric and Helga insisted we protect the children, even though I knew just one of them would have been enough. I wish I could have stopped your sister…or at least found a way to stop Mordred. Godric…he never liked bastard children, which is the pot calling the kettle black. I haven't found her corpse. Otherwise, I would have her a proper burial. Bastard or not, traitor or not…I think she deserved that much. She was to be pitied, not hated. But…you didn't even give her that. I'd like to think she's how our daughter might have turned out. Maybe less violent, but certainly as rambunctious, strong-willed, and with a fire Godric would have been proud of."

No reply was forthcoming, and he sagged. "Ria…already they're singing songs about you. They have been for some time, but I know the fucking bards are going to be singing about your glorious final battle…not knowing or caring that so many people have died. That you were once a sweet little girl, cursed by the blood of your father, with a destiny. That you weren't actually happy with Guinevere, not in the way that we were. That I lost you when you pulled Caliburn from the stone. I would give almost anything, even my magic, to have you amongst the living once more, and free from that damned destiny. To be the person I knew you should be. A strong woman, yes, a warrior woman…but not a king. You killed your heart and soul doing that, and mine as well."

He stood, looking down at the corpse of his beloved, noting how the sunlight made her hair look like woven gold. He wished he could see her eyes, emerald like his own. "…Ria…Godric and I had one hell of a row about Merlin. When I made some remark about Merlin being a manipulative cambion, Godric accused me of being a Blood Purist. Stupid hotheaded bastard. Unfortunately, that nephew of mine most certainly is, and is already corrupting the students I've given him to tutor. They're going to be using that incomplete version of Kaleidoscope to take Hogwarts to another world. But…I've had enough. I don't want to teach anymore. I'm not even sure life is worth living now. Not…not without you, Ria." He sobbed quietly and openly.

Then, he straightened, getting off the ground and looking at the corpse of the woman he loved. "There's one thing I can do, even if I die trying. I'm going after that bitch of a sister of yours. There were many reasons why that civil war happened, like your style of ruling, that mess with Lancelot and Guinevere, rejecting Mordred as an heir…but in the end, Morgan set fire to the kindling. Helga told me I should be merciful to the defeated…and look where it got us. No. Sal's gone, dead with his sense of mercy. Salazar Slytherin has taken his place. Goodbye…my love…"

And with that, he walked away from that sun-dappled wood, leaving behind the corpse of the Once and Future King, and the woman he loved…

It had been a dream that he had had for some time, one he generally only-half-remembered when he woke. Certainly, he never remembered who the man was. The odd thing was, the dreams that haunted him, they felt more like memories than dreams.

His eyes opened rather blearily, to meet a pair of crimson ones with slitted pupils, vaguely cat-like, and certainly looking at him with cat-like curiosity. Said eyes were the only strange feature in the face of a beautiful girl, perhaps in her late teens or early twenties, with blonde hair, wearing a white jumper, and a long dark skirt. "So, you're awake," the girl said, smiling. "You gave me quite the scare, you know. I thought you were one of Grandfather's apprentices."

"…Sorry, what?" he asked. He seemed to be lying on a sofa in a living room.

"Oh, right. Grandfather said you'd be confused. It's not every day that an accidental congruence of different types of magic emulate one of the True Magics," the girl said. "That's what he said, anyway. You dropped right out of thin air in my apartment, along with a corpse and a trophy. It's where I stay whenever I'm not hibernating or hunting down Roa."

"Hibernating? Roa?"

"Right, Grandfather said you probably wouldn't know. He was able to read some of your surface thoughts. Your name is Harry Potter, right?" Harry nodded. "Well, I am Arcueid Brunestud. You can call me Arc if you like."

"…Okay, Arc," Harry said, before he suddenly realised. He'd just seen Voldemort resurrected, Cedric killed, the duel, and then, while escaping, a curse hit the Portkey. Panic surged in his chest, and he tamped it down with an effort. "Your grandfather, is he…is he a wizard?"

"The Wizard Marshall of Clock Tower, actually, and the master of the Second True Magic, Kaleidoscope," Arc said proudly. "He's not actually my grandfather. Harry…I should point something out to you. It's going to be a bit of a shock, so brace yourself. You're not on your version of Earth anymore."

Harry's eyes widened. "…What?"

"Yeah, I know it's hard to believe, but…well, when you've been around my grandfather as long as I have, you have to keep a fairly open mind."

"And who's your grandfather?"

"You probably won't have heard of him. He is Kischur Zelretch Schweinorg, though most people call him Zelretch."

The name indeed was unfamiliar to Harry. Instead, he focused on what needed to be done. "Okay, well, Arc…thanks for looking after me, but…I need to find a way to get back home. If what you said is true, then I need to get back. He's back, and nobody knows, and…and…" Panic began welling up in him again. Voldemort was back, he was, if they were telling the truth, stranded on another world.

"Hey, calm down," Arc said, putting a hand on his shoulder.

"Don't tell me to calm down!" Harry snapped, trying to get off the couch, only to realise that Arc was holding him in place, with a strength her slender form should not have possessed. "He's probably attacking my friends even as we speak, and you're telling me to calm down?! I've had a really bad year, to be honest. No, scratch that, most of my life. So don't tell me to calm down when I have no reason to. And let go of me!" He tried to take her hand away, pulling at her arm as much as he could, only to realise, he couldn't.

"If I wanted to, I could hold you here all day," Arc said, her red eyes peering into his own. "You want me to let you go? Make me."

Harry obliged with a Banishing Charm that sent Arc flying, though she landed on her feet, barely missing some furniture. "That was very rude," Arc said rather casually. Suddenly, she seemed to blur out of existence, only for Harry, who had stood up after hitting her with the Banisher, to hear her behind him. "And just remember, human, that it pays to be polite to the White Princess of the True Ancestors. A little bit of friendly advice."

"White Princess of the what?" Harry asked, whirling to find her sitting on the very sofa he had just vacated, looking amused and irritated at the same time.

"Bluntly, I'm a vampire. On this world, there are two types: True Ancestors, and Dead Apostles. True Ancestors are the most powerful kind of vampire, born rather than made, and don't need to drink blood to survive. My grandfather is a Dead Apostle. Unlike most of our kind, we're not predators on humans. Well, not for blood, anyway. My grandfather likes hearing tormented screams from those he pranks, though."

Before Harry could figure out what to say to that, the door opened. "My ears are burning, my dear Arc," a deep, gentle and mirthful voice said. The man whom it belonged to looked elderly, dressed in robes, with a beard, and crimson eyes. However, there was an undeniable sense of power about the man, but also a gentleness and a joviality. He was both alike and yet unalike to Dumbledore.

"…You're Zelretch?" Harry asked.

"Indeed. A pleasure to meet you, Harry Potter."

"I've told him that he's on another world, Grandfather," Arc said. "He's…rather upset. He's worried for his friends."

Zelretch waved his hand dismissively. "Don't be. I am aware in broad strokes of what is happening in your end of the multiverse, Harry. Knowing Voldemort, he will spend months at least gathering his followers to him, months that you can use to get better."

"And how can you know this?" Harry demanded.

"Because, aside from looking through your memories, I am the master of Kaleidoscope, the Second True Magic, one of the only ones still in existence to this day in this world," Zelretch said. "Kaleidoscope deals with parallel worlds, including the viewing thereof, and the travelling between them. In truth, I could get you home very easily. But, and please do not interrupt me, what's the point? As you are now, you would be an irritation to Voldemort. You need to become better and stronger, for the sake of protecting your friends. I can help you there."

Harry's eyes narrowed. "What's the catch? And no, I'm not turning into a vampire or Dead Ancestor or whatever it is."

Zelretch laughed. "No, no…nothing of the sort!" His expression became rather solemn. "But it is dangerous, mortally so. And while I cannot force you to do this, it's quite possible that the fate of the world rests on this. I'd like you, Harry Potter, to take part in the Fourth Holy Grail War…"

It was some weeks later, and Harry was wondering whether he should have disagreed more vehemently, instead of allowing Zelretch to persuade him. After all, he had already just dealt with one potentially lethal magical tournament. To get involved in another (and voluntarily, even!) seemed like the height of insanity. And yet, here he was, in a park, at midnight, in Fuyuki City in Japan, watching as a dark-haired boy a few years his senior drew out a complex magical ritual circle. "For the origin, silver and steel. For the cornerstone, gem and the Archduke of Contracts. For the ancestor, my great master Schweinorg. The alighted wind becomes a wall. Close the gates in the four directions. From the crown, come forth. Trace the three-forked road leading to the kingdom. Fill, fill, fill, fill, fill. Repeat fivefold, and when each is filled, destroy it."

Harry sighed quietly. Waver Velvet was a bit like Hermione, only more egotistical. But in their short acquaintance, they actually managed to get along fairly well. Waver was a student at Clock Tower, the main HQ of the Magus Association as well as their main school. Waver had published a paper that had drawn the ire of Kayneth Archibald, Lord El-Melloi, a haughty man who looked like Lucius Malfoy, only with more brains and a better haircut. Apparently, Waver had posited that a Magus of a less-established family could match the power of a more-established family's Magus through hard work and discipline. Here, magic was generally stronger the longer one's family had been practising it, though Harry noted that there was still a similar elitist snobbery that he despised.

Waver had stolen something from Lord El-Melloi, an ancient scrap of red cloth that apparently belonged to Alexander the Great, aka Iskandar. Harry only learned of this later, when Waver was halfway to Japan, and Zelretch used Kaleidoscope to dump him at Waver's destination. Waver was determined to fight in the Holy Grail War, and Harry, after much persuasion from Zelretch, decided to join too. In the end, he was doing it to try and save lives more than anything else, something that wasn't generally considered by most Magi: as long as they could cover it up, a loss of life was acceptable.

And there was something else, something deep within him, that called on him to do this. A yearning to see someone again, for some weird reason. But still, he knew he was in the deep end here.

Nearly two centuries ago, three of the top Magi family in the world, the von Einzberns, the Makiris and the Tohsakas, pooled their resources to create a tournament that was, theoretically, capable of reviving the Third True Magic, Heaven's Feel. But in order to do so, for some reason, they needed to set things up in a strange way. Basically, there was something called the Holy Grail, not the drinking cup of Jesus Christ, but rather, a magical artifact that could be charged with a specific type of energy. That energy came from the essences of Servants, a type of familiar that was used to battle to the death between these Magi. And not just any familiar, but the revived essence of heroes from myth and history, known as Heroic Spirits. In short, the Holy Grail War was basically a tournament between seven heroes from history and myth, as well as the Magi they were in service to.

Zelretch, with a knowing smile, had given Harry something that, he claimed, 'fell off the back of a lorry heading to the von Einzberns'. It was a beautiful wedge of metal, seemingly made of gold with blue patterns and ancient writing. It was called Avalon, and was allegedly the sheath of the legendary sword Excalibur. The reason for this was that, apparently, your odds of getting a specific Servant increased if you used an artifact associated with their life. Waver was going to go with Iskandar, and Harry, it seemed, was going to get King Arthur.

"Well, I'm ready," Waver said. "I'll go first, okay?"

Harry nodded, and he stood back as Waver began pumping his mana into the magic circle, his hand outstretched. "Heed my words. My will creates your body, and your sword creates my destiny. If you heed the Grail's call, and obey my will and reason, then answer my summoning! I hereby swear that I shall be all the good in the world, that I shall defeat all evil in the world! Seventh Heaven clad, and the great words of power, come forth from the circle of bindings, Guardian of the Scales!"

There was a massive flare of light, and as it died away, Harry blinked in astonishment. The man who appeared was tall and burly, his skin tanned, his hair a fiery red. He was dressed in Grecian armour with a red, fur-lined cape draped over his shoulders. "Servant Rider has answered your summons. I ask of you," he said in a gravelly rumble of a voice, "are you my Master?"

Waver, who was staring up at his Servant in shock and awe, eventually nodded hastily. "Yes! I'm your Master! Waver Velvet!"

"Hmm. Then the pact is sealed. Though what's this? Another Magus? Is he your associate, or competition?"

Harry glared at the Servant. "I'm Waver's ally, but I will be summoning a Servant of my own. So, would you mind leaving the circle for the moment?"

"Ha! Of course! It'll be interesting to have an ally." The giant of a man strode unhurriedly out of the circle, and Harry handed the scrap of cloth back to Waver, before taking Avalon and placing it down. Then, he began the ritual himself.

And that's when it happened. Even as the circle dissolved into an actinic flare of light, he felt something within his mind burst, like a collapsing dam. And then…he remembered.

Waver's eyes widened as, just as the circle flared, Harry collapsed. Now, Waver hadn't known Harry for all that long, and he wasn't sure whether he could call the sardonic teenager a friend, but to see him collapse to the ground and start convulsing was not good, especially when he vomited. "Harry!" he screamed.

Suddenly, the figure he could barely see within the magic circle dashed out, supporting Harry. It was a girl (or else a VERY androgynous boy), perhaps about Harry's age, with blonde hair framing regally beautiful features. Her emerald eyes, so much like Harry's own, were in concern. She was dressed in what looked like a blue armoured dress, complete with breastplate and gauntlets. Her bearing seemed older than her apparent age suggested.

"What sorcery did you inflict on him?!" the girl demanded.

Waver held his hands up. "We didn't do anything! He was summoning you, and then, suddenly, he just collapsed!"

The girl searched his face for any deceit, before nodding. She then looked at Harry, who was gasping painfully, before his eyes flickered open. "Are you all right, my Master?"

"I feel like dragon shit," Harry rasped, before his eyes widened. "…Ria?" he asked, in a disbelieving tone of voice. He reached out his hand to caress the girl's face. "But…how…oh…that's what they did…Heaven's Feel, in an incomplete form…"

The girl's eyes widened on being called 'Ria'. Then, she seemed to peer closer at Harry. "…Sal? But…how can this be?"

"Ro' would probably know, little swot," Harry said, before scowling. "That fangfaced bastard knew…I'll bet you Arondight that he knew, or suspected. Arturia…you're back…"

Arturia nodded, a sad smile on her features mirroring Harry's own. Somehow, improbably, a reunion had been engineered, across centuries and dimensions. But time would tell whether it would last…

CHAPTER 1 ANNOTATIONS:

So, Harry's been inveigled into another magical tournament…but he's Salazar Slytherin? And he knew Arturia? Well, I'm sure many of you will take this as proof I've gone off the deep end.

No numbered annotations this time.