Ty madripper for being my beta; found an impossible to spot typo and helps me with the plot

Shirou took a deep breath before engulfing his right arm with darkness. It turned completely black and acquired pointed tips on all his fingers.

Whilst it did look somewhat creepy, all it did was add a 'Heartless' attribute to his arm, allowing him to extract a heart with it without breaking the skin.

No less fatal, but within his Realty Marble it was the best he could hope for to minimise the risks to Sakura. Shirou took a deep breath before he plunged his right arm into her chest. It went right through her clothes and skin like a ghost before touching her heart.

The Heartless attribute activated, instantly separating her heart form the rest of her arteries whilst his Reality Marble activated and put Sakura in stasis. Shirou extracted her steaming heart with not even a drop of blood spilt. The worm came along for the ride.

'There is that disgusting thing!' Shirou thought angrily as he used his human left hand to extract the worm and crush it. Usually that would rip her heart to pieces but the Heartless attribute was still in effect, allowing him to extract the worm through the heart using his human left hand.

It squealed before it was crushed in his Reinforced hand.

Sakura's heart was now being engulfed in a black light due to being prolonged contact with his Heartless hand.

'Shit; better act quick before Sakura becomes a Heartless without a body!" Shirou plunged his right hand back into her chest and deposited her heart back into its correct position.

"Dark Heal!" Shirou yelled as he channelled his Prana into Sakura's body through his Heartless arm. There was no light show as the magic was already internalised in Sakura's body.

Dark Heal, along with his other Dark spells – he somehow knew that he had others –, were instinctual spells that only appeared in Shirou's head when he needed them.

However they would only 'appear' within his Reality Marble and were forgotten – repressed? – after the Reality Marble ended. Not that it made much of a difference; Shirou instinctivly knew that the World hated his Dark Spells to the point where it would attempt to erase him from existence if he tried to use them.

And this wasn't the standard 'to be a magus is to walk with death' stuff either. Shirou was absolutely certain that he would die if he chose to use his Dark Spells outside of My Heart of Darkness.

Dark Heal wasn't a super healing spell without risks though. It only worked on those from the Dark Realm, meaning that Shirou was balancing Sakura's life on a knife's edge. He was gambling that there was enough darkness left, from his arm, on Sakura's heart that Dark Heal would be effective enough to reconnect her heart before it all faded away. This conclusion left him reasonably confident as well as trusting his instincts.

Unknown to Shirou, Dark Heal did more than heal Sakura; it activated the digested pieces of the Holy Grail from the Fourth War. Those pieces had been consumed by a Crest Worm; a worm that had replaced forty percent of Sakura's nerves with its own body.

The substitution was complete though, meaning that forty percent of Sakura's nerves functioned as both a nerve and a high quality magic circuit. With no brain from the Crest Worm in Sakura's heart to tell it what to do; it had now become a proper internal Magic Crest.

A Magic Crest that was now without a guiding intelligence and unleashed the 'Shadow': a near formless entity that hungers for magic; made up of pieces of Angra Mainyu.

If this had occurred within the Earth, a connection to the Great Grail would have formed. The small amounts of Angra Mainyu, contained within the pieces of the Grail, would have linked up to its main body in the Great Grail.

Angra Mainyu would have then attempted to bring itself into the world through the slaughtering innocents for their Prana with this method.

But this activation didn't occur within the Earth; it occurred within My Heart of Darkness. And thus as the darkness of the Shadow enveloped a healing Sakura, Shirou recognised a fellow instinctual being.

"A dark power sleeps with your heart, Sakura…"

'Hearts are the source of all power…'

"Very well, darkness exists to feed and devour instinctual beings like this. Here, have everything…"

With a gesture of his still free left hand; Shirou directed the darkness around them; which shivered, before flowing into the Shadow trying to consume Sakura and his arm. With a sudden new source to feed upon, Shirou's right hand was released from the hungry grip of the Shadow. He immediately jumped back to observe the rest of the phenomenon.

The small pieces of Angra Mainyu still present within the rest of the Magical Crest were being consumed by the everlasting hunger of darkness; which even now began possessing the Magic Crest around Sakura's heart and central nervous system. An explosion of dark prana erupted from Sakura's body, easily triple Shirou's output.

Shirou looked at this explosion and smiled. This was the successful result his instincts were telling him about. As he watched Sakura with his Structural Grasp he witnessed the dark prana eating every remaining worm inside Sakura's body, cleansing her.

With a sudden pulse, his Reality Marble shattered and Sakura's dark covered form started moving. Shirou took off running at her, determined to comfort her if she was in pain.

He lifted her up bridal-style from the cold floor and gazed upon her.

When he used his Structural Grasp on her he was struck speechless: Sakura's Magic Crest was connected directly to the Realm of Darkness.

'This is incredible, she functions more like a living portal to the dark realm then a Grail at this point. What further progress could be made if she studies the Matou magic now? She could be the key, the one with the ability to summon Heartless into this world. No, I am sure that she is the one; dad said the Matou specialised in familiars. It was only that old vampire's desire for immortality that ruined their Magecraft.'

Shirou turned off his Structural Gasp and examined Sakura's physical appearance with his normal eyes. Sakura's violet hair had turned white and what he had initially assumed to be a dark power around her was actually a black cloak; one that had overwritten her school uniform. Sakura stirred and opened her eyes cutely. Wine red orbs instead of violet; still without a pupil.

"S-senpai?"

'Well she sounds the same at least.'

"Sakura, I was so worried. But the operation went flawlessly." Shirou smiled down at Sakura and kissed her forehead.

"Ahh, S-senpai… Thank you." Sakura gave him a true smile in return. As he examined her with a bit more Structural Grasp he saw the darkness pulsing out from her Crest.

The Crest was forty percent of her nerves and would cause her pain when used. But it was now the same as any other Magical Crest from other mage families. Maybe a bit more painful as the Crest was her nerves. But no consciousness was there; Shirou was certain of it.

Thus Sakura was safe from the control of that worm-ridden corpse of a grandfather and it was time for her to secure her freedom.

"Sakura, you have to secure your future now." He emphasised the seriousness of this conversation.

Sakura looked up, slightly confused. She was still blushing from the kiss but with each pulse of her Magic Crest a feral presence made itself clearer. Shirou smiled; 'This may be easier than I thought. Might as well be blunt then.'

"You must purge your home of all life so that you can never be hurt again; Sakura... So that we can one day get married and… so no one can hurt our children." Shirou looked straight at her, completely honest as he told her to kill her family.

'They're just future hearts after all…'

Sakura looked up at her Senpai as if for the first time. Then she smiled at her perfect senpai as her darkness pulsed to the beat of her heart.

"Hai, Senpai." A Dark Corridor opened and she vanished from his arms.

Shirou was stunned speechless; not at the display of teleportation magic that he couldn't use outside of his Reality Marble. It was Magecraft that far surpassed his own.

No, he was surprised at the ease that Sakura said she would kill her family for him.

He thought long and hard, before laughing. Just an empty laugh, devoid of any emotions.

'She views me as perfect. Absolutely perfect; therefor any command I give her must be the best one…

Heh, we are both mad… mad beings of darkness trapped in the Realm of Light.

Don't worry my love; we shall create a Realm of Darkness for us to belong… Is the love we have madness? Is it a strength or a weakness? Will the World be able to stop us? I must find these answers…'

When did the laughter become tears?

Never the less, Shirou finally allowed himself to collapse on the cold ground. He had overused his circuits to perform complex and incredibly sensitive magic; before feeding his entire output to the Shadow in order to connect it to the Real of Darkness.

It had then changed the nature of Sakura's Magic Crest.

He knew without a doubt that Sakura's storage capacity was immense and she would have an infinite hunger for magic. But that was ok. An infinite hunger had met and connected to an infinite source thanks to his efforts. And Sakura had been saved. Now it was time to rest…

Sakura arrived home to an almost empty house. Almost; as it had a lone insect still residing in it.

"Sakura, what are you doing here you bitch?" Shinji Matou, rapist and her adopted brother, said without turning around from the kitchen; raiding the fridge.

'KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM…'

The darkness in her heart pulsed with that single-minded thought. Shinji, Grandfather; everyone had defiled her. But Shirou cured her. Her Shirou was perfect. He was without flaws. Any mistakes he made would be due to no fault of his own.

'Like leaving Shinji alive. Shinji might hurt Senpai if he is left alive… KILL HIM… And then it would be my fault that Senpai died. KILL HIM KILL HIM KILL HIM KILL HIM KILL HIM…'

"Sorry, Ni-san…" A group of dark tendrils rose out of her shadow.

*Squelch...*

"S-sakura…you bitch…" A body fell. The dark tendrils withdrew back into her shadow.

Sakura was sad. She should be sad. So why couldn't she stop smiling…?

"Oh, it's warm Ni-san…" She brought the steaming red organ to her mouth…

Zouken Matou was doing something which he had never considered doing in his long life: running away from Sakura.

'I am probably the only person in the world who could receive information from inside Emiya Shirou's Reality Marble. Although that's not exactly correct; I received my soul instantly once the Reality Marble was dropped.'

His main Crest Worm, the one containing his soul, was inside Sakura Matou's body. That one was destroyed, with his last sight being Emiya Shirou's hand crushing it. Once the Reality Marble was dropped his soul moved to his other lesser Crest Worms currently infesting his corpse body. They were lesser anchors but his soul was undamaged; only its main anchor was destroyed.

This meant that he was reduced to being an actual finite existence, since he couldn't jump to anymore bodies without his main anchor in place. He also couldn't create another Crest Worm unless he regained the Matou Mansion and the worms within the cellar. And he couldn't beat Sakura as she was now; meaning he had months at best to live.

Every last worm familiar in that house had been consumed by Sakura in an instant after she killed Shinji. Every last drop of Prana lost and every worm shrivelled and dead. He had the worms inside his body and that was it.

'The Holy Grail War better start soon so I can summon a Servant. I still have some tricks left even without a reagent to summon one with… I… WILL… LIVE… FOREVER…'

One week later…

Things had finally settled down from after that nerve-wracking night. Sakura had put some of her blood and Shinji's memories into Shinji's corpse, turning it into a corpse familiar rather than a human one. This familiar was now attending school after a week off due to being 'sick'.

Rin was probably the only one who could tell the difference; and she obviously wanted to confront Sakura about her brother's death. But for some reason she never did; especially after seeing Sakura cling to my arm, smiling serenely – or was that a smirk? – at her whenever she came close.

Someday Shirou was going to ask about the history between them.

Never mind the fact that after only a week of studying, Sakura had created a successful familiar. This made Sakura just as smart as Rin in his opinion. Not that Shirou had any idea how competent Rin was as a mage; but he trusted his Instincts.

Shirou's Structural Grasp also told him that Rin was almost without equal: She had Forty… FORTY naturally high-class Magic Circuits within her body along with 30 high-class ones from her Crest. In comparison to Shirou's twenty-seven high-class circuits and an average magus's 20 average-class circuits she was a ridiculous monster of a mage.

On a side note, Shirou found it ironic that as a familiar; Shinji Matou had the Magic Circuits he always dreamed about.

Unfortunately Matou Zouken had escaped into the night; though much reduced according to Sakura.

Shirou sighed and kissed Sakura's head, causing her to snuggle. He could wait no longer to fulfil his second objective: Rescuing Illya from the Einzbern family.

Shirou's Prana had recovered and Sakura had just told him that she had secured her mansion's Bounded Field. With wishes that she kept safe and continued her catch-up studying; Shirou spent the rest of the day at school; and told her the he wouldn't won't be coming back to the Matou until late. Taiga-sensei – don't let her catch you saying that! – would only accept 'starvation' for so long. And since Shinji was 'all better' now; Shirou had no excuse not to return home to feed her. 'Honestly, feeding my sensei sounds a lot more romantic then what it really is: throwing good food at a ravenous beast.'

None the less he lasted the entire day and prepared for what could be the most dangerous night of his life sans the fire that started it all. After feeding the beast and calling Sakura – and getting wished good luck – Shirou dressed up in an all black outfit consisting of a black polypropylene undershirt, black coat and pants with black military silenced boots and a white snow jacket.

He brought with him a first aid kit and a pair of white goggles for the snow. The point of all these blacks and whites was to hide in the snow forest just outside the Einzbern Bounded Field whilst bringing it down. Once the Bounded Fields were dome he would drop the white clothing and envelop the entire castle with My Heart of Darkness.

Although there was no natural light within the Reality Marble; due to the lack of knowing Illyasviel's location Shirou would have to envelope the whole castle. This meant that light provided inside the castle would still be available.

The good news was due to it being such a large area the usefulness of Dark Corridors increased.

The bad news was once again the area. Shirou was good; but he was no Rin or Sakura in terms of Prana. He had a minute tops in this situation. Even worse, he had no familiars to scout; as Sakura barely finished Shinji in time. The prudent thing would be to wait for another week to create – or recreate – some Blade Wing Worm familiars used by the Matou to scout. But Shirou would not allow Illya to be without her family for another second now that he could help.

"This heart belongs to darkness…" Saying the beginning of my aria, Shirou partially manifested his Reality Marble and opened a Dark Corridor. He took one more breath and jumped into the darkness.

Germany, The Einzbern Castle…

"Now Illyasviel, you will be summoning your Servant today. This is so you can get revenge on Emiya Kiritsugu by killing his son Shirou; who he replaced you with." Jubstacheit von Einzbern spewed his poison into little Illyasviel's ear. It was two months before the start of the Fifth Holy Grail War and Jubstacheit would accept nothing more than total victory. They planned to summon the strongest Servant in the strongest class: Heracles, also known as Hercules, as Berserker.

To pull this off the Einzbern family had modified Illyasviel's magic circuits to the extreme, allowing her to generate immense amounts prana to the point that she could summon and maintain a corporal Berserker without the aid of the Grail. This was also aided by the Einzbern Wish-Granting Sorcery Trait; which allowed her to skip all thaumaturgical processes and go straight to the result; so long as the person had enough prana. (Aka fuck the process; give me results)

Of course this drastically lowered Illyasviel's lifetime: it was guaranteed that she wouldn't live past twenty. Not that she would appear twenty at that point in time either; rather she would die before puberty organs were developed.

She was already eighteen yet she appeared twelve at the oldest.

"Yes Grandfather. Do you have a reagent?" Illya innocently looked back at her Grandfather; who showed her a massive box on the floor.

"Yes, now prepare yourself Illyasviel. If you fail you will be cast out; I do not accept failure." With his harsh words said; Jubstacheit left Illya in her room. Her was her parents' old room and on one of the top floors overlooking the grounds.

Gazing through the window; Illya was the first person in Germany to see a Dark Corridor manifest itself just outside the Einzbern Bounded Field.