Fate/Stay Night belongs to Kinoko Nasu and Type-Moon

Date A Live belongs to Koishi Tachibana

This story takes place during the Heaven's Feel route

Chapter 1

A Freak Accident

It was about time.

In the dimly lit, though quite spacious basement of the Matou household, only two people could be seen. One was a small, shriveled old man standing in front of a magic circle, and the other was a blue-haired youth that was more off to the side of the room, reading a light novel to pass the time. The first was named Zouken Matou, and the other was his grandson, Shinji Matou.

Zouken would begin chanting an incantation, as the magic circle in front of him begun glowing a bright red. He was summoning a Servant, which would be considered pretty odd, as all 7 were already accounted for. However, one of them wasn't summoned... properly, and thus, he had an opening to bring in one of his own. As the incantation finished, there was a bright red flash in the centre of the magic circle... but nothing was there.

"Wait..." Shinji would peek over his book to check whether the summoning was successful, only to see the big bunch of absolutely nothing in the magic circle. "Where's the new Assassin? I thought that the Servant always appeared in the circle!"

"That would be the case in a typical Holy Grail War." Zouken responded to Shinji's... actually quite reasonable question. "However, there can only be one Servant of each class at any given time. Because of that, I can only imagine that the fake summoned by Caster will probably be replaced by my true Assassin soon enough."

The only thing that Shinji could do was simply accept Zouken's reasoning and keep on reading his book. After all, he was no magus, and was thus useless in a proper Holy Grail War... as much as he hated to admit that.

Assassin was, as usual, quite bored. He was to simply stand guard by the temple gates, fighting any intruders that may come around. This, of course, wasn't helped by the fact that he literally could not leave them behind to actively seek out a fight, as his summoning method caused him to be bound to the gate itself, unable to go anywhere too far from it because of it. He hated every second of it, and wished that his Master, the Caster Servant of this war, would just find a way to unbind him already so he could go out and fight. Then again, she probably wouldn't, as he was literally just another familiar to her that happened to be way more powerful than her usual ones. Despite how necessary she was to his survival, Assassin only had one thing to say to himself. That being, "I really do hate Caster."

But then, all of a sudden, he felt a sharp pain in his chest that caught him off-guard. It felt like something was writhing in his ribcage, trying to burst out towards its freedom, along with a non-stop series of what felt like papercuts assaulting him from within. Eventually, whatever it was flew out of his chest, leaving a multitude of holes in his torso that dripped blood. When he looked up, he saw multiple bloody pieces of paper floating in the air, which he could then swear shifted into more humanoid silhouettes before his vision went all black.

Suddenly, Caster began losing her connection to her Master, her beloved, Souichirou Kuzuki. Not wanting to lose him, she immediately warped to his location, but when she opened the doors, she was met with a sight that could only be described as"not pretty".

Kuzuki was lying down on the floor, with someone touching him quite intimately and the area covered in blood. The person in contact with him appeared to be a young girl, with black hair that faded to white at the tips, done up in thin low twintails, and blue eyes with small white pupils that Caster could swear were faintly glowing. The girl wore what appeared to be a black, white, and gold nun's habit at around the length of a miniskirt, with a proper white skirt underneath to cover her bottoms and somewhat detached skin-tight sleeves that left her shoulders uncovered, as well as tall black boots that went up to her thighs. When Caster looked closer, it turned out that the girl's hand that appeared to be in contact with her dear Souichirou was actually inside of his body, and it was very clearly not simply phasing into that position, if the blood on his clothes in that spot was any indication.

"Would you look at that? It's the one we were waiting for." The girl spoke in a creepily cheerful tone, compared to the grim actions she was undertaking. "Hello, Caster! How is Assassin doing?"

"What interest do you have in Assassin? Let my Master go!" Caster prepared to attack, but that was quickly stopped by the girl wagging her finger in front of her.

"Mm-mm-mm~ If you try to attack me, then your Master dies. But... he's not just your Master, is he~? Is he, perhaps... a lover of yours~?" She switched to a more teasing tone, as if to play with Caster's feelings before doing whatever it is she wanted.

Caster was shocked at how quickly the girl saw through that. He kept his relationship with her a secret, as he should in order to preserve the mystery of magecraft. Did she torture the information out of him? Either way, she knew she couldn't just attack if she wanted him to live, so she let her hands down.

"There we go~! Now then, if you want your beloved Master to live, I would recommend for you to use your Noble Phantasm on yourself. I know what it can do, after all. Cutting away any magical contract... even the ones between Masters and Servants... I can't perform my duties if held back by some pesky Command Spells, you know?" Those words confused Caster more than anything. How did this girl know about her Noble Phantasm? Why would her Command Spells be this much of a problem for her?

"W- What are you?" That was the only thing Caster could ask to this person.

"I'm not telling you anything just yet. Now then..." And it was then that an ultimatum was given to Caster. "Your Command Spells, or his life. Choose wisely~"

Caster then realized she had no other choice. If she didn't do this, then Souichirou would die. So, she brought out what the girl wanted. It was a thin, brittle looking dagger, with a twisted, thunderbolt-like blade that had a rainbow sheen to it. This was her Noble Phantasm, Rule Breaker, which she then plunged into her own chest. A surge of red energy enveloped the room, as the Command Spells vanished from the back of Caster's hand. As the lightshow settled down, the girl sighed, smiling at Caster.

"Hehe~ Thanks for setting me free, Caster~" She sounded genuine, but even then, something felt off to Caster.

"Now then... What are you?" Caster asked once again, but this time, she actually got a response, and not one she was expecting.

"Oh, right, now that you did that, we can actually tell you! We, lady Caster... are the true Assassin of this war." The answer was terrifying in a way that defied explanation. "We"? And did she just say she - or they - was/were the war's true Assassin? What in the hell did this mean?

However, at that point, she felt a sharp pain in her gut, and as she looked down, she saw a familiar, albeit bloody hand poking out. The hand of the girl. The hand of the so-called "True Assassin". But Caster could swear that the girl was still next to Souichirou... pulling her hand out of his body, it covered in his blood. The sight would've been nauseating, if not for Caster's own experience with sights like these and the excruciating pain that coursed through her body. She knew that the moment that True Assassin pulled her hand out, she would die, as she didn't have any skills such as "Battle Continuation" to keep her alive after receiving such a wound, and the girl - or girls, as it appeared - probably wouldn't allow her to heal herself with magecraft.

"You know, you should've known that summoning a Servant while being a Servant yourself was a risky move." True Assassin took some time to mock Caster, who was then restrained by what appeared to be two more of her. Were those copies? Was the Servant in front of her some sort of hivemind? The fact that she even had to ask these things terrified her even more, as she had no idea what else True Assassin was capable of.

"Don't worry though." The True Assassin behind her stated, in the same mocking tone that was already getting on Caster's nerves. "We won't just let you up and die. You're still quite useful to our Master."

It was at that moment that the pain in Caster's abdomen spiked, and she momentarily blacked out.

Zouken, having just sent some of his crest worms out to infest Caster's body and take control of it with help from the Shadow, widened his eyes in shock, which was clearly noticed by Shinji, who was still reading his book.

"What happened, Zouken? Did something go wrong?" He asked, quite obviously worried about Zouken despite his... issues.

"...Yes and no." Zouken replied, a tinge of conflict being heard in his voice. "The Assassin is doing what they're supposed to, but on the other hand, it's not the Servant that was supposed to be summoned. I was supposed to summon one of the Hassan-i-Sabbah, and instead got a bunch of little girls in nun's habits..."

"...little girls in..." Shinji muttered to himself before checking the cover of his light novel. Indeed, the character portrayed on this particular volume matched Zouken's description. "Uh... Zouken, do they have black hair fading to white and generally act really smug?"

"Yes. How did you know that?" Zouken wondered, curious as to what made Shinji ask that. His curiosity became even greater when Shinji dropped his light novel on the floor, his face showing an unmistakeable degree of shock.

"No... No, this is bullshit!" Shinji stood up and punched the wall in frustration, too much so for him to feel the pain from such an action. "I thought fictional characters could never become Servants, especially not ones from such a new piece of fiction! Why the hell is everything going so off the rails!?"

"Fictional-" The old man would then notice the book, and quickly piece together what happened. "...I see... So that book you were reading was picked by the summoning ritual to be the catalyst... This could either be really good for us, or absolutely terrible."

"What do you mean it only could be terrible!? It is terrible!" Shinji continued his... quite justified temper tantrum. "The Assassin summoned by Caster was a Phantom, and your so-called "True Assassin" could only possibly be that too! Forget amongst Servants in general, they'd be the weakest even amongst Assassins!"

"Calm down, Shinji." Zouken said with an unmistakeably authoritarian tone of voice, making Shinji follow the order on principle, before settling down and explaining himself. "Yes, they may individually be quite weak. However, whatever weaknesses they may have will be unknown to the other Servants, as the Holy Grail doesn't deem such fiction relevant enough for the war. Not to mention, their state as an apparent hivemind could be quite useful in reconaissance, especially when combined with Presence Concealment."

"Heh... You've got a good point, old man." Shinji had no options but to accept the explanation.

"Now then..." The old man in question had an evil smirk adorn his face. "Let us see what we can do with this freak accident."

A/N:

So yeah, we have the Nibelcoles replacing Cursed Arm Hassan as True Assassin. As such, I'm gonna give you a stat sheet, which will be given again every chapter in which an update would occur.

True Name: Nibelcole

Class: Assassin (labeled as True Assassin due to Sasaki Kojiro's prior summoning)

Alignment: Chaotic Neutral (leans towards "Chaotic Evil" due to the Master's influence)

Class Skills:

Presence Concealment C

Personal Skills:

Hivemind Page A

A representation of the connection between the individual True Assassins. Unlike the Assassin of the Fourth Holy Grail War, True Assassin is a hivemind connecting multiple bodies indistinguishable from one another in appearance or manneurisms, as opposed to multiple varied individuals born from a single being. They do not have what could be called a "main body", as every single True Assassin is the real one in equal measure, and thus, they can only be truly killed if all of them are destroyed.

??????????????

Noble Phantasm: Currently unknown

Yeah... Trust me, I thought this through quite well, even if not much is said. Once again, it'll be expanded upon as stuff is revealed. And yes, the different abilities between Cursed Arm and Nibelcole will indeed cause some pretty big changes.

Next chapter: Lancer and Assassin