The Realm of Darkness, a realm that mirrors our own. Not in shape, but in composition. For every bit of light across every world, darkness in equal amounts was present in this realm. It was a realm with no concept of time or reason, filled with the gas/liquid/solid known as darkness; enough to blanket all worlds. There was no intelligence, no life, no Type-Darkness existed. The only existence within this realm was the same existence that was in all realms: Akasha, the Swirl of the Root; also known as Kingdom Hearts.

Angra Mainyu was the being closest to darkness in the Realm of Light; yet it too would fall far short; being a sum of all the world's evils. Pure darkness was absolute instinct, it would consume all those weaker than itself, which was why no intelligence had yet to be formed and it had no concept of 'sin' or 'evil' to base itself around.

However three almost simultaneous occurrences would open up a brief door to darkness and change its nature forever.

First; during the Fourth Holy Grail War; the Lesser Grail was destroyed whilst overflowing. After only absorbing five of the seven Servants; the Lesser Grail was full. This was due to death of Iskandar, also known as the King of Conquerors: Alexander the Great; as well as the death of an unusually powerful Berserker.

This was hardly the first time the Lesser Grail had been destroyed; otherwise there would be an actual winner in the previous wars; but this was the first time it had been full.

Second; before its destruction Angra Mainyu had actually interacted with and cursed a Master. A Master had actually refused a wish!

Angra Mainyu had been furious; here it was; full power and beating at the doors of its prison, and it had been refused. Its scream of rage and frustration sent shockwaves similar to the ones a screaming Gaia had released four thousand years ago. But rather than be answered by the Types from other planets; the Realm of Darkness heard it.

But it didn't have a true intelligence; there was no Type that could manifest itself on Gaia. It needed a summoning.

And thus the third event: five hundred souls sacrificed and consumed by darkness. A corridor to darkness was opened within the flames; and five hundred screaming souls were devoured. This imprint created the first existences of the dark realm; five hundred new beings known as Heartless. Being of pure instinct and hunger; they poured right back across the corridor and into the world.

Only to encounter the mother of all roadblocks: the World. The Heartless tripped just about every single warning both Alaya (the will of humanity) and her opposite Gaia (the will of the Earth) had and they were not letting them through. Gaia opposed them due to being alien and Alaya opposed them due to the high chance of them leading to human extinction. They needed a back-door into the world whilst Angra Mainyu was still going strong and the corridor existed.

The soon to be Emiya Shirou was an empty existence. In order to survive the flames of his burning home he had thrown away all the things that held him back. He had no memories anymore; he had thrown them away. Nor did he have emotions; as they held him back from taking another step forwards.

Shirou didn't know this; but he had turned himself into a Nothing; a pseudo-Nobody. Rather than losing his heart; he had simply emptied it out completely. Instead of his heart being an existence of light and darkness; his heart was a devoid of the smallest speck of darkness and the smallest glimmer of light. Thus the darkness of five hundred damned souls flowed in.

Shirou didn't scream, he couldn't scream, as he collapsed; his hair turning white and his eyes turning yellow. The instinctual powers of the Heartless immediately activated his twenty-seven Magic Circuits and overcharged them. The pain was excruciating as the body of Emiya Shirou was overridden by five-hundred other existences taking residence inside his heart.

If left alone; the Heartless known as Emiya Shirou would emerge, probably as a Darkside. Shedding its mortal coil it would rampage whilst its very existence was opposed by the World. It would eventually fade away or be destroyed and the world would just tick over. But Kiritsugu Emiya found him and inserted Avalon, the holy sheath of King Arthur, into his body.

An artifact of supreme healing and light cloaked his body and healed it. A heart belonging to the Realm of Light is simply light with a dark core. Even an evil man would simply switch his heart's nature. Deep down there would still be a light that never went out.

Emiya Shirou now was a core of darkness surrounded by light. Kiritsugu Emiya had succeeded in multiple things that had never been accomplished before with one simple action: He had saved Shirou's life, body and mind. He had fooled the world itself into thinking Emiya Shirou was a natural existence. And he had disguised a heartless as a human. The world now treated Shirou himself as a heart.

Ten years later…

Emiya Shirou was an odd magus. Not incompetent, as everything he showed even the slightest affinity to he mastered, so quickly it was almost instinctual. The problem was the fact that he was only shown to have an affinity with Reinforcement. He always used Structural Grasp in combination with Reinforcement; and could Structural Grasp with a glance. In fact, Shirou was half convinced he was born with a unique Mystic Eyes of Structural Grasp; due to his yellow eyes.

But his fighting instincts were inhumane. Oh he didn't master weapons and martial arts as soon as he picked up the said weapon; but he learnt all forms of combat and weaponry fast. Shirou didn't know this; but due to being an entity of instinct at his core; he was in some ways similar to a certain famous King of Knights from the previous Holy Grail War; and was slowly developing the Instinct and a budding Eye of the Mind (False) skills.

A normal Eye of the Mind is the 'snatching victory from the jaws of defeat' from experience gained of getting out of or succeeding in such conditions across many battlefields. Battlefield experience on steroids is an apt comparison.

Eye of the Mind (False) is similar, but instead comes from something artificial and is usually focused on getting out of a said situation rather than succeeding in it. Rather than having experience; something has supercharged your survival instincts to get out of the said situation. Spiderman's 'Spider-Sense' would be the most obvious comparison.

If Shirou kept up his progress, the modern world was going to have a hell of a Magus Killer v2.0 on their hands. He may even become a Heroic Spirit after death; probably a Saber, although with his recent progress a Caster wasn't far behind.

Kiritsugu had been startled with the weird 'talent' that Shirou had; as if he was designed to be his apprentice. Shirou didn't have any talent in creating Bounded Fields (also known as wards if one was a Harry Potter fan). He did however seem to have an affinity to pierce and destroy them with ease.

This was fundamentally different to Kiritsugu's method of modifying and exploiting holes in Bounded Fields and thus going in undetected. Shirou had talent, actual talent, in being a sledgehammer. Kiritsugu found it ironic.

Thus he told Shirou everything; about the previous Holy Grail War and more importantly; about his daughter Illyasviel von Einzbern. And the secret horror he had at her being turned into the new vessel for the Lesser Grail in the next war. He also told Shirou of his inability to assail the Bounded Field of the Einzberns due to him advising them on their weaknesses. He had done this whilst still adopted in their family as a branch member.

Kiritsugu believed that since he had defeat himself; and the Einzbern's Bounded Fields themselves were ridiculously strong; only someone like Shirou could actually rescue his daughter. But going up against the Einzbern homunculi was suicide normally, so Shirou would have to prepare for a war or sneak in. The instinctual Heartless inside of him thrived in such conditions. Training, Combat, Mission. All Heartless had a Mission; and all Heartless sought out new hearts.

Shirou was still unaware of what he was; all he knew and cared about was that he learnt even faster now that he had an objective, the logic coming from his human body telling his instinctual self that he wasn't strong enough.

Later on he had also met a girl whilst training: Sakura Matou. From the moment he had met her; he desired her.

Of course; Emiya Shirou was a gentleman; as taught by Kiritsugu. But the instinct of a Heartless; which was frantically telling him that she was an ally; was confused with human hormones. And thus Shirou rationalised that he desired Sakura even from a young age. His Structural Grasp and life sensing instincts told him of the horrors inside that poor girl's body. Thus a wish was created. A wish to save two girls with his power.

A wish that in his deepest instincts became an absolute order: Emiya Shirou will save his sister and his mate. Emiya Shirou will save his family. This broken, distorted thing inside of him interacted with Avalon and began crafting a Reality Marble from the blueprints.

From this was born a distortion far less damaging to himself; but far more damaging, in the future, to the World.

It was the end of a Friday, the last day of the week. Girls crowded around two boys: Shinji Matou and Shirou Emiya. Both had exotic looks, Shinji with his blue hair and eyes; Shirou with his white hair and yellow eyes.

However Shirou was a lot bigger; as expected from the kendo and archery champion of the school. He had finally managed to beat his teacher, kendo instructor and legal guardian, this year. Her name was Taiga Fujimura, daughter of a Yakuza boss and still receiving an allowance.

Shinji and Shirou had been friends for a long time until one event happened that killed it off for good: Rin Tohsaka for some reason had a crush on him. Or at the very least, was fascinated by him. Always touching, poking and prodding Shirou. Come to think of it; she was probably the main reason that he even had girls coming after him. He wasn't that great…

But the pretty girls after him were the last thing on Shirou's mind. He had finally completed his trump card, if it could be called such a thing. His earnest wishes and desperation for a solution after his father died had led to research in the most forbidden of subjects: a reality marble. It was Magecraft close to Sorcery that overwrote the rules of the world and substituted in your own. A forbidden Magecraft due to the high chance of it driving the people insane as they desperately tried to see the world from a different angle.

Shirou didn't care; his was going to save his family.

He could not fight the entire Einzbern household and rescue Illya; so the world had to change into something that allowed him to do so. He couldn't take away the Crest Worm in Sakura's heart without killing her, so he had to imagine a world where he could. A world of darkness; where beings could survive without their hearts and darkness so complete that it could hide him completely in order to rescue Illya. That image and those rules came rather easily to him and thus he created an aria; containing the truth about himself and the world.

It would allow him to cure Sakura Matou; and thus allow them to be together. There was no room for doubt, as the rules changed to make him successful.

"Sakura-chan, please accompany me home today. I have some good news." Shirou told his cute junior as he walked through the school hallway. She nods and replies with a "Hai, Senpai," before accompanying Shriou with a smile.

That smile stayed on even when she spotted Rin Tohsaka, who stood in front of us smiling naturally. Of course Shirou's instincts were yelling 'false' at him, so he knew that the smile was a mask. 'She probably wants to kiss and dissect me; and not guaranteed in that order.'

But as Shirou was a gentleman; he will once again play the fool to Rin. "Oh, Tohsaka-san, is there something you need?" Shirou said with a smile. Rin smirked cheekily at him before glancing over at Sakura; and suddenly she looked slightly sullen. "Nothing Emiya-kun, I was just wondering what you are doing for the rest of today." She smiled for the invisible camera and Shirou's instincts once again screamed out 'fake' to him.

Suddenly Shirou was assaulted with a massive migraine; and held his right hand up to his temple in an attempt to quell it. Words appeared in his head and Shirou's headache increased in severity.

'Seven Princesses of Heart are needed…'

"Gaaaaah!" Shirou grits his teeth in pain. His head feels like it's about to split open.

'…to open the door to Kingdom Hearts…'

Just as suddenly as it appeared, the headache vanished.

Although Tohsaka obviously suspected something; due to a light frown she had on her face, she wouldn't make a scene of it at school. Taking a deep breath, Shirou smiled once again at Tohsaka. 'My instincts and magic sense told me that she was a magus and I am pretty sure she suspects me. It's either that or she genuinely wants to cut me up for doing better than her at school. I think it's frankly terrifying that it may indeed be the genuine reason she is after me. Damn tsundere personality traits; you could never tell.'

"Ahh, Senpai! Are you ok?" Sakura worries over me, running her hands over my shoulders. 'What a wonderful girl. I better take her home with me and cure her - was that a grope?'

"Don't worry about me Sakura-chan," Shirou smiled crookedly at her. "Soon, we will be happy together."

"Ahh, Senpai…" Sakura shook her head and looked very shy; blushing horribly and sneaking glances at Shirou. 'What a cute junior.'

"Hmmm. Shirou, you better not be doing something weird to Sakura!" Rin crosses her arms and looks away, the angry-but-not-angry tsundere superior look.

"Ahh, don't worry Tohsaka-san. I will take good care of our cute junior. She will be my future wife after all." Shirou shamelessly announces his intentions.

"…"

"WHAT!" Rin enters her super tsundere mode and Sakura's entire face turns red. Steam even seems to rise from her head and she looks down at the floor.

"S-senpai…" Sakura's quick look turned hungry before it disappeared; much to Shirou's secret satisfaction. 'It was always the quite ones. Rin seems to be charging up for a rant but the fulfilment of a promise I made was much more important.'

"Come on Sakura. Let's go home." Shirou smiled and reached out with his right hand.

"Hai, Senpai! Goodbye Tohsaka-senpai," She smiles and 'was that a glare at Rin? No, I must be imagining things.'

Rin was left speechless. Well, not exactly for long…

Back home Shirou prepared for the most complex sequence of events and magic of his young life. This was to be the first of three planned times he was going to use his Reality Marble for a ritual. He couldn't make a single mistake. No, Emiya Shirou would not make a mistake. He had very few objectives; none could be allowed to fail.

He took Sakura to his workshop. At this point it didn't matter if she knew that he was a magus or not. If his instincts were right she would soon be the last Matou alive anyway.

"Alright Sakura-chan. I need you to trust me. I will cure you; right now, I promise." Shirou gave Sakura a winning promise. He was going to fulfil his ideal; right here, right now.

"Hai, Senpai!" Sakura nodded at him whilst saying her catchphrase. She had the utmost faith in her Shirou.

Being his future wife…ahh...she felt lightheaded...

Shirou sighed as he watched Sakura's fallen form. 'Well at least she knocked herself out.'

He closed his eyes and began his aria. The incantation that would always be unique to him and the key to his hopes and dreams.

"This heart belongs to darkness…" He knew that from the moment he was saved from the fire.

"All worlds begin and end in darkness…" Such things were only logical.

"This heart is no different…" The darkness of death never faded.

"Darkness lives within it; it grows, consumes it…" He was a creature of instinct above all else.

"Every heart returns to darkness…" Every heart belongs to the realm of darkness.

"Thus this world has been connected

Eclipsed by my heart of darkness!"

A shining wall of black light shone from the ground up, expanding in a circle; showing the border of the Bounded Field. Within his My Heart of Darkness there was only the absence of everything. For nothing of this world could survive in the dark realm. Unfortunately anything brought into it by Shirou, friend or foe, was invisible to this corrosive effect. It had no weapons inside, nor an actual attack.

But one of its properties was 'absolute isolation'. It was a completely different realm, and based on King Arthur's Avalon, although he wasn't aware he had it inside of him, and even further isolated when compared to other Reality Marbles. Nothing from the outside world could reach it. That included the Will of Gaia; for if even the slightest rupture existed; the realms of Light and Darkness would be locked in eternal struggle and annihilate each other.

Magic in here could be performed without Gaia interfering. Not even a Command Seal could remove a Servant from his realm.

But it was the one rule that existed in this this realm that would allow him to save Sakura. One impossible rule: That you could live without your heart in this realm. With this one rule; he would be able to remove the Crest Worm from Sakura without killing her.

It was time to get to work…