Return to Zero: A Fate/Stay Night Fanfic Idea
It is the closing phase of Heaven's Feel, and Emiya Shirou is on the brink of death. Having defeated Black Saber, freed Sakura from the grip of Angra Mainyu, and outlasted Kotomine in their final showdown, there is just enough energy left in his body to project Excalibur and obliterate the Holy Grail. However before he can do this, he is stopped by Ilya, who sacrifices herself using the Third Magic on him and transfering his soul into her body (The cavern the Grail is housed in collapses soon afterwards, destroying the Grail anyways). Thus Emilya Shirou is born, and returns to Sakura and the gang alive and well, if also turned into a ten year old girl.
However what should have been a happy ending is interrupted several months later by the sudden appearance of multiple Dead Apostles (Nasuverse vampires, for those not familiar with Tsukihime) in Fuyuki City. Drawn by the residual power of the completed Black Grail, at least one Dead Apostle Ancestor--perhaps Nero Chaos or Altrouge Brunstud--shows up in Fuyuki and begins wrecking havoc. Emilya, Rin, and Rider go into action against the vampires, but before the final battle is over, Sakura has been captured and placed in mortal danger, and Emilya is forced to make a pact with the world as Archer did in order to gain the power to save Sakura from enemies of vastly superior power. This she accomplishes, but at the cost of her life. So it goes that Emiya Shirou becomes Counter Guardian EMILYA.
Now rewind the clock eleven years back.
Kariya Matou is the only decent human being left in the Matou family. He is Shinji's father and Zouken's descendent, and is the last magus in the Matou family, the last family member with the hope of restoring the family's power. However at a young age he decided he wanted nothing to do with Zouken or the family's foul magecraft, and left the family to its' fate. Now the Fourth Holy Grail War is right around the corner, and Kariya has just learned that the daughter of his beloved Tohsaka Aoi, Tohsaka Sakura, has become trapped in Zouken's vile plans for the family's resurrection through a deal with Tohsaka Tokiome, Aoi's husband.
Having no desire for young Sakura to suffer at Zouken's hands, Kariya offers Zouken a deal; if Kariya participates in the Fourth Grail War as Zouken originally wanted him to, and Kariya wins, then Zouken will agree to release Sakura back to her true family. Zouken accepts this deal, but because Kariya has not had training as a magus, Kariya will have to become a host for Zouken's crest worms to augment his power and become capable of using magecraft and summoning a servant.
When Kariya summons his servant, Zouken instructs him to summon a Berserker-class, a wrecking ball suitable for smashing through anything that may get in his way. In the original Fourth Grail War and the events of Fate/Zero, Kariya summoned Lancelot as a Berserker and lasted most of the war before being defeated in the final stages when the crest worms consumed all his prana and his life.
This time, however, while going through the motions of the summoning procedure, Kariya's thoughts are occupied by the inevitable fates of Aoi and Sakura, and his summoning is marked by a wish, one that repeats itself over and over in his head like a mantra:
...Save Sakura...
...Save Sakura...
...SAVE SAKURA....!!
There is a flash of light, a roar of wind, and the surprised yelp of a little girl dropped unceremoniously on her ass in Kariya's summoning circle. His heart drops into his stomach as he beholds his "servant"; a little girl with pink hair dressed in a cute schoolgirl's uniform. He doesn't even need to think about it too hard to realize he's failed; there are no Heroic Spirits at that age, and certainly none that would be wearing a modern-day primary school seifuku. He must have botched the summoning and produced some sort of familiar...
...although, it's a rather strange familiar that comes equipped with a stone ax four times her size....
Kariya has summoned Counter Guardian EMILYA as Servant Berserker, the only Servant Class besides Saber that Emilya would qualify for due to her unstoppable determinator behavior when her loved ones are in danger. Unlike Herakles, Emilya is not constantly insane thanks to one of her Noble Phantasms, which allows her to suppress the Mad Enhancement and function normally until she needs ME's stat boost in battle. This is very good news for Kariya, as it means that Emilya is much easier to maintain most of the time than Lancelot would have been, and his prospects as a Master have just risen considerably, though he doesn't know it yet because he doesn't believe he's actually summoned a Servant.
For her part, Emilya is thrilled at the prospect of having the chance to save Sakura before she ever suffered under Zouken's treatment, but in order to accomplish this he has to claim the tainted Grail in Zouken's name, something he absolutely will not do under any circumstances, knowing as he does that wishing on the Grail will result in total destruction of the human race. Caught between a rock and a hard place, Emilya and Kariya struggle to find a way to save Sakura from Zouken while also stopping the other Masters from claiming the Holy Grail and dooming humanity. Kiritsugu, Saber, and Irisviel may be of assistance with this, but there's no guaranteeing that the Magus Killer will trust them enough to not shoot Kariya on sight, and Saber is too focused on claiming the Grail and reversing her past to open her mind to Emilya's warnings.
However, regardless of the consequences Emilya is determined to find a way to save Sakura one more time. Preserving her smile is the only reason she exists anymore, and nothing and no-one will stop her from achieving that goal. Not Kiritsugu, not Saber, not Kotomine, not Gilgamesh, and especially not Zouken.
Look out world, there's a new anti-heroine on the block!
However what should have been a happy ending is interrupted several months later by the sudden appearance of multiple Dead Apostles (Nasuverse vampires, for those not familiar with Tsukihime) in Fuyuki City. Drawn by the residual power of the completed Black Grail, at least one Dead Apostle Ancestor--perhaps Nero Chaos or Altrouge Brunstud--shows up in Fuyuki and begins wrecking havoc. Emilya, Rin, and Rider go into action against the vampires, but before the final battle is over, Sakura has been captured and placed in mortal danger, and Emilya is forced to make a pact with the world as Archer did in order to gain the power to save Sakura from enemies of vastly superior power. This she accomplishes, but at the cost of her life. So it goes that Emiya Shirou becomes Counter Guardian EMILYA.
Now rewind the clock eleven years back.
Kariya Matou is the only decent human being left in the Matou family. He is Shinji's father and Zouken's descendent, and is the last magus in the Matou family, the last family member with the hope of restoring the family's power. However at a young age he decided he wanted nothing to do with Zouken or the family's foul magecraft, and left the family to its' fate. Now the Fourth Holy Grail War is right around the corner, and Kariya has just learned that the daughter of his beloved Tohsaka Aoi, Tohsaka Sakura, has become trapped in Zouken's vile plans for the family's resurrection through a deal with Tohsaka Tokiome, Aoi's husband.
Having no desire for young Sakura to suffer at Zouken's hands, Kariya offers Zouken a deal; if Kariya participates in the Fourth Grail War as Zouken originally wanted him to, and Kariya wins, then Zouken will agree to release Sakura back to her true family. Zouken accepts this deal, but because Kariya has not had training as a magus, Kariya will have to become a host for Zouken's crest worms to augment his power and become capable of using magecraft and summoning a servant.
When Kariya summons his servant, Zouken instructs him to summon a Berserker-class, a wrecking ball suitable for smashing through anything that may get in his way. In the original Fourth Grail War and the events of Fate/Zero, Kariya summoned Lancelot as a Berserker and lasted most of the war before being defeated in the final stages when the crest worms consumed all his prana and his life.
This time, however, while going through the motions of the summoning procedure, Kariya's thoughts are occupied by the inevitable fates of Aoi and Sakura, and his summoning is marked by a wish, one that repeats itself over and over in his head like a mantra:
...Save Sakura...
...Save Sakura...
...SAVE SAKURA....!!
There is a flash of light, a roar of wind, and the surprised yelp of a little girl dropped unceremoniously on her ass in Kariya's summoning circle. His heart drops into his stomach as he beholds his "servant"; a little girl with pink hair dressed in a cute schoolgirl's uniform. He doesn't even need to think about it too hard to realize he's failed; there are no Heroic Spirits at that age, and certainly none that would be wearing a modern-day primary school seifuku. He must have botched the summoning and produced some sort of familiar...
...although, it's a rather strange familiar that comes equipped with a stone ax four times her size....
Kariya has summoned Counter Guardian EMILYA as Servant Berserker, the only Servant Class besides Saber that Emilya would qualify for due to her unstoppable determinator behavior when her loved ones are in danger. Unlike Herakles, Emilya is not constantly insane thanks to one of her Noble Phantasms, which allows her to suppress the Mad Enhancement and function normally until she needs ME's stat boost in battle. This is very good news for Kariya, as it means that Emilya is much easier to maintain most of the time than Lancelot would have been, and his prospects as a Master have just risen considerably, though he doesn't know it yet because he doesn't believe he's actually summoned a Servant.
For her part, Emilya is thrilled at the prospect of having the chance to save Sakura before she ever suffered under Zouken's treatment, but in order to accomplish this he has to claim the tainted Grail in Zouken's name, something he absolutely will not do under any circumstances, knowing as he does that wishing on the Grail will result in total destruction of the human race. Caught between a rock and a hard place, Emilya and Kariya struggle to find a way to save Sakura from Zouken while also stopping the other Masters from claiming the Holy Grail and dooming humanity. Kiritsugu, Saber, and Irisviel may be of assistance with this, but there's no guaranteeing that the Magus Killer will trust them enough to not shoot Kariya on sight, and Saber is too focused on claiming the Grail and reversing her past to open her mind to Emilya's warnings.
However, regardless of the consequences Emilya is determined to find a way to save Sakura one more time. Preserving her smile is the only reason she exists anymore, and nothing and no-one will stop her from achieving that goal. Not Kiritsugu, not Saber, not Kotomine, not Gilgamesh, and especially not Zouken.
Look out world, there's a new anti-heroine on the block!