First of all, let me introduce myself. I am Célian Cogitore, author of this Fanfic. I do not own the rights of Fate/extra nor those ofAoki Hagane no Arpeggio.I don't really speak English. This is a machine translation, just reworked. I bet the result is clumsy. But I'm not a specialist. As for the idea of crossover... how to say it ? A.I. + A.I. and then I am a member of the Church of Saint Hakuno. And I think it's time she made some new friends... don't you? Edit : some minor rewitting.

Lunar Depth 00

The Moon Cell, also known as Eye of God and the Holly Graal, is an immemorial recording device construct by some sort of quasi divine extraterrestrial beings. This massive collection of photonic crystal within the moon is a sort a supercomputer.

The humanity discover this artefact in the year 2030. But the creation of the Moon Cell is anterior by far... 4, 6 billion of years, in fact.

Who could understand Moon Cell?
Time for him does not have the same meaning as for man. Paradox of its nature, it considers that a thousand years is a brief moment, and yet performs in a second calculations that would take several lives.

The Moon Cell observed the Earth from the time it was only an ocean of molten lava to the time when - a rock deprived of life- it was swallowed by the sun, turned into a black hole. Several times. In different time lines. This supercomputer saw the appearance of life. Its proliferation. Great extinctions. And the man... from the first step until he disappeared.
The creators of the Moon Cell did not give it its own consciousness because it could interfere with its purpose: to observe. To intervene would be to leave his impartial role.
Neither good.
Neither bad.
All he does is observe.

Monumental library, the Moon Cell recorded everything. In the .Ph., the virtual space that constitutes the inner universe of the Eye of God, lies the lunar chimeric seas. In their depths is stored all that has existed, all that will exist, coming from several divergent time works of art, the first forms of recorded life, the ruins of civilizations that have not yet appeared, the Mayan cities at their peak... all... floating in a kind of primitive broth.

Then appeared the Spiritron-hackers, Magi capable of hacking computers by using their magic circuits.
He allowed them to enter the .Ph in the form of avatars and to access some of his knowledge. The mere fact of observing them taught him enough for the Moon Cell to welcome them.
Soon he left his usual position as an impartial observer.
Why did he open the Holy Grail Wars?
Who the hell knows?

In any case, they offered him the opportunity to watch the humans fight through Servants. Heroes of the past to whom he had given a virtual body.
Perhaps seeing the greatest heroes in human history allowed him to learn more about them. Perhaps the tactics and strategies invented by the Magi offered him a direct observation of the best and the worst of the human soul.

There were many who dared the adventure. After all, even if only one of them survived, he could ask the Holy Grail to grant the wish that would present with the incredible power of this giant computer, its knowledge of the past, the multiple futures it could have seen, what was really impossible? The victor of the Holy Grail War could save the world from pollution, restore natural resources... or become the immortal tyrant of a slave humanity.
Nothing was impossible.

Many times the cycle of the Grail War was repeated, without victory.
Because the fragile mechanism of this conflict had been subtly distorted by an A.I. The real Twice H. Pieceman had been a doctor hating war. But to populate the Grail Wars, the Moon Cell had created NPC, digital copies of living humans, charged with making the daily lives of the participants bearable.

By a strange aberration, Twice developed his own consciousness. He took part in a Holy Grail War and won it... to discover that he could not enter the Main Core of the Holy Grail without being erased. The Moon Cell recognized it as a set of corrupted data, a bug that had to be removed before it would infect the entire network. Nevertheless, unable to violate his own rules of non-intervention, he could not attack the Master Twice H. Pieceman.
Wars followed one another.
The winner arrived in front of the Main Core to face Twice H. Pieceman.

The latter asked the Magus to grant his wish in his place: to plunge humanity into an endless war so that it can evolve... according to the Darwinian principle of survival of the fittest. Because Twice really was a corrupted avatar of the original.
The magus wouldn't do it.
Twice was killing him.
It was all starting again.
Again...
Once again... ...
Again... again... again...

The final battle against Saver... the Buddha himself... had been gasped, exhausted for having resorted to the Unlimited Blade Work, his Noble Phantasm. Twice had been defeated, agreeing to disappear. Because, according to the principle of survival of the fittest, he had just demonstrated his own incompetence.
Rin was gone, back to Earth.

Apart from the Servant with white hair and silver eyes, there was only a thin girl left. Dressed in the brown uniform of Tsukumihara Academy, and black pantyhose. She must have been sixteen years old with long wavy brown hair and eyes. His expressive features showed nothing... the kind of character that remained in the background and was forgotten.
Yet she was the victor of the Holy Grail War. She had survived the seven weeks of this war, crossed the fourteen levels of the chimeric oceans, defeated the seven Masters and their servants... and even defeated twice Julius Harway who had not accepted his own death...
More, she had achieved the impossible by beating the undefeated Saver!

Her name was Hakuno Kishinami.
At the last step of the staircase, the teen girl touched the golden sphere in the center of the Main Core cube.
She disappeared and found herself floating in a sea of data.
The moment of her triumph.
The moment of her death.
Indeed, the case of Hakuno Kishinami was no different from that of Twice H. Piereman, she was a N.P.C. created by the Moon Cell to populate the benches of Tsukumihara Academy.
Irony.
Cruel irony.

Hakuno had no wish. She had gone through the war with no other purpose than to survive. But the very rules of the Holy Grail prevented her from doing so. The Moon Cell destroyed corrupted data that reached its Main Core to prevent an infection of its programs.
Discreetly, the Servant had followed his Master in his Astral form. Nameless enveloped her with his presence as she faced her fate courageously. She bit her lips, containing her tears.
- I want the Grail Wars to end. I do not want there to be a single death for the Grail. I want all the alterations that Twice created, to cause humans to wage war on each other, erased.
Hakuno closed his eyes, waiting for his end.

Was it his punishment for killing the other masters ? Destroyed their dreams ?, she thought,
Even her original hadn't had a chance. All Hakuno knew about the original girl, whose virtual copy she was, was that she had survived a horrible war before becoming incurably amnesiac. She had been immersed in cryogenics and probably still had to wait for the treatment that would cure her. Which would never happen. The Harway consortium that controlled the Earth had banned technological progress.
This is not fair! I have not asked to participateto this massacre.
Her Servant sent her thoughts of courage to comfort her. He was determined to save her. But Nameless didn't really hope to be able to do more than delay her disappearance by more than a few seconds...

If the Moon Cell had had a conscience, he might have been saddened or annoyed by the situation. As a computer devoid of emotion, he merely noted the unsatisfactory conclusion of the Grail Wars.
The last of them, since the winner had just used his wish to forbid their repetition.
Hakuno Kishinami would therefore be the only winner of the Grail War.
Or to be more exact, in all time lines, 158 variations of Hakuno Kishinami (53 male and 105 female), Master of the Servant Archer (Namless) or Saber (Nero Claudius Germanicus) or Caster (Tamamo No Mae) had been the only winners. And each of these variations had made the exact same wish: to prevent any new Grail War.

Logical machine, the Moon Cell did not reject this conclusion.
Only he couldn't accept it either.
There was a deep reason behind the Grail War. The wish was just a way to attract the most talented Magi to compete in the .Ph, to find the best of them.
The Moon Cell was then to offer him the Regalia, a program that took the form of a ring worn by the avatar of the winner. The ring would make him the Suzerain of the .ph. The role of the Suzerain was to make decisions necessary for the survival of the .Ph. Indeed, a danger threatened him: Velber, the Umbral Star an another entity of photonic crystal with a goal contrary to those of the Moon Cell. Velber plundered the civilizations of the galaxy to steal their knowledge, using force to fulfill his purpose. For the Umbral Star, the Moon Cell was like a natural prey, full of knowledge... Devoid of proper consciousness the Moon Cell could only defend itself automatically and, without imagination, unable to win. He needed a Suzerain to organize his defense with the genius of a Grail War winner.
Except, the Moon Cell was in front of two rank-one priorities:

- Destroying the Hakuno Kishinami program, this NPC suffers from a bug and is in the Main Core, threatening to corrupt the fundamental software of the Moon Cell.
- Protect Lady Hakuno Kishinami so that she can defend the Se. . against Velber.
Logical machine, the Moon Cell had to apply both instructions since they had the same priority level and that any breach would cause its destruction. Except that they were mutually exclusive. An irresoluble paradox. Maybe, it could have a moment of weakness...
The Moon Cell left an opening in its defense system. Nameless spotted this weak spot in the firewall surrounding the Main Core and struck with all its force, creating a rift. Warned by his Servant, Hakuno rushed outward... transforming into a stream of data flowing into a sea of possibility.

/ Designated Moon Heir/
/Grail Wars Winner/
/ Suzerain /
/Hakuno Kishinami/
/ Regalia, in transfer... /
/ Transfer 25%.../
/ Transfer 55%.../
/ Transfer 75%.../
/Transfer Completed/

The MN (Marine Nationale) Surcouf was a submarine cruiser... an original idea of French engineers, seeking to create a warship capable of sailing under the waves. More importantly, circumvent the 1922 Washington Treaty, which limited the size of battleships. Since it did not touch the size and armament of the submarines, it seemed a good idea.
Put on hold in 1927, launched two years later, it had only been armed in 1934...
The construction had been difficult. The engineers encountered almost insurmountable sealing and air conditioning problems.

After the humiliating defeat of 1940 against Nazi Germany, the MN Surcouf joined the France Libre of General de Gaulle, continuing the fight. It was then the largest submarine in the world, capable of firing 203mm shells on the surface and equipped with twelve torpedo tubes and two anti-aircraft guns.
He disappeared at sea... in Bermuda. Victim of the infamous Devil's Triangle ? Or a Catalina bomber who took it for a German submarine? Will we ever know?

The years passed. The war ended with the bombing of Hiroshima and Nagasaki. The world changed. Between 1996 and 2006, nearly 59% of the western Antarctic ice melted. Coastal cities were submerged.
From 2012, witnesses described World War II warships, appearing all over the place. Initially treated with contempt, this information ended up being taken seriously as warships from different states saw it in turn.
In 2038, Russian ships attacked one of the "ghost fleets" that had penetrated its territorial waters. The result was the complete destruction of the Russians. Their opponents used futuristic weapons: energy rays, guided drones, missiles. Russian fire was stopped by impenetrable energy screens. Worse, the ghosts ships engaged the "corrosion torpedoes", weapons creating a real localized black hole, or "supergravity cannons", having the same effect. Nothing that humans had created could protect them from such weapons. The hulls were swept away, torn apart like paper.
In December of the same year, a battle pitted the united fleets of all the nations of the world against what was beginning to be called the Fog.

The defeat of the humans was complete. Only one submarine, the I-401, was captured by a commando. Analyzed for nearly ten years, it turned out to be made of nanomaterial and controlled by an artificial intelligence.
In 2058, humans no longer dared to venture into the oceans. Planes no longer flew over them, destroyed by the Fog's ships. The coasts were filled with impressive defences. But famine ravaged the most isolated countries, especially islands and archipelagos.
Slowly, the world of men collapsed as they began to kill each other for the remaining resources.

The Fog version of the Surcouf looked like the original. A one hundred and ten meter long vessel, for a displacement (diving) of 4,218 metric tonnes. The emerged part of the ship was painted in grey two tones, the hull in the kiosk one could read "17 P". The submarine was most recognizable for its 2 x 203mm turret.
Of course, he was just regaining the appearance of the original ship. He was using very advanced technologies.
Despite its large size and powerful armament, the Surcouf was not highly regarded in the Fog navy operating in Atlantic. Despite the high technology of the twin of the real Surcouf, he was not treated any differently from other submarines.

In the first place, the submarine had received only a simple core, far inferior to the core delta that fitted the admirals ships... not to mention that they had two or even three of them.
The great revolution of the Fog - the Mentals Model- had been refused to the Surcouf.
What was a Mental Model?
An android created by the same nono-machines that formed the ships of the Fog. It received the Core and became the "commander" of the boat. The Mentals Model had above all the ability to experience feelings and emotions, based on the human model (hence their name). However, it had a price... a decrease in performance that could be very clear to the simplest Core. Originally only the battleship Yamato had a Mental Model, and then it allowed battleships, cruisers, aircraft carriers, and other large vessels to have one.

Devoid of emotion and feelings, Surcouf continued his patrol off the coast of England. His objective was Brest, in french Brittany. He was authorized to engage any vessel or human aircraft approaching him. Yamato, on the other hand, forbade any attack of humans in the interior.
An ordinary patrol, no events.
And then...
System alert, data corruption... alert... malfunction... losses... da... ta... critical failure... computer attack... hacking... fire... wall... Ineffective...
For a few moments the lights went out on board the Surcouf.

The interior of the ship, plunged into darkness, was only smooth and bare surfaces. The central station had only a different element, a black cube with a kind of round eye. Like the rest of the submarine, it was made up of nano-machines. It began to melt, turning into a silvery puddle, animated by violent twitches. The foreign program which had taken possession of the "brain" of the Surcouf sought to return to its normal form.

Slowly he adopted a humanoid aspect which gradually lost its appearance of liquid mercury, to take the tones of the flesh. The long hairs turned brown. The being had even reconstituted his clothes, a school uniform also brown.

Hakuno Kishinami opened her eyes... in complete darkness. She wished for the light to come on and... on board the Surcouf, her wishes were orders. Hakuno discovered a dome-shaped room with white walls and a single corridor behind her.
In her mind, she visualized the whole ship, as an extension of her own body... she even saw the outside of it. Feeling something in her hand, she looked at it to discover a ring. As she directed her thought on the latter, instinctively seeking contact, she discovered... all the software running the Surcouf, literally digested by the Regalia. Virtual windows opened in front of her, with curves and images, as well as lines of technical explanations. The ring was asking for instructions to steer the ship!
- What is going on ?
In disbelief, the teenager shook her head. Her last memory was the Moon Cell, waiting for her to be erased and there...
- I became the commander of an underwater cruiser ! ?