Securing the allegiance of a trained assassin was just another step in Illya's preparation, but it was a massive boon to all of her plans already. Souichirou was a mysterious man who made her wonder about the details of his past, yet he told her just enough for Illya to realize that he was able to help her. As the two were on their way, Kuzuki explained more about his fighting style and how Caster's magic empowered him; Illya was certain she could create a similar effect with her large reservoirs of mana; all they needed was a little practice.
Speaking of where they were going…Illya led Kuzuki to the woods. He was cautious at first, like anyone in his situation would, but Illya explained the purpose of their swift voyage. What they needed most was information about Zouken resting deep within von Einzbern archives. Of course, the only complete version of those archives was all the way back in Germany, but Illya's old castle, which she used during the grail war, had its own library with important information about the world of the magi. Some of it Illya barely paid any mind to, and it was a mistake she desperately wanted to correct.
"This was your base of operations, then?" Kuzuki said as he stepped inside the forest castle's main hall. This place has gone empty with Illya and her maid tending for it, but the girl did not regret it. The place had outlived its usefulness by this point; why would she waste her energy on it? It reminded Illya of all the chains von Einzberns put on her, memories far from happy. Heracles was the only one who made this place lively and provided Illya with fun company. Hopefully, this will be the last time she has to step inside.
"Wow, you're a tough nut to crack, Kuzuki."
"Pardon?"
"You see a literal castle in the middle of Japanese woods, and that's all you have to say?"
"Did you want me to get excited?"
"Most people would…Ah, whatever, that's not what we're here for!" Illya exclaimed and rushed up the stairs. "Come on and keep up, I'll show you the library section!"
"Very well," Souichirou said and followed her. He was so stoic and expressionless that Illya couldn't help but chuckle at his complete lack of antics. He was similar to Heracles in that way, observing silently and following her lead. Just like with Heracles, Illya could see a form of desire and emotion buried deep within Souichirou, but he was a master at masking it.
"I see your family has spread its influence far beyond their homeland," Souichirou noted as he looked over the place.
"The Fuyuki city was there where the Holy Grail is buried, so of course, they needed an outpost here. I won't be surprised if someone else from their ranks takes over it in a couple of years…Angela or Heinrich, probably."
"Some of the German magi?"
"Yes, not particularly pleasant, especially Angela…" Angela was among the ones who oversaw Illya's training. Back then, she was a maturing mage who needed to prove her worth to the family and did everything to get the elders' approval. "I hope I won't…Well, hopefully, I won't have to see this chaos play out. And here we are! Welcome to my personal library."
Illya and her companion entered a large room full of dusty bookstalls. Illya spent quite a lot of time in here, though she never bothered cleaning it. Playing hide-and-seek with Heracles was her favored activity.
"What exactly are we searching for here?"
"Anything that will paint a clearer picture of what Zouken is. My 'family' had a whole lot of spies; they heard rumors here and there. Now, where was it…" Illya rushed over to the library, jumping up the nearby ladders if she had to. It took a long time for her to find anything noteworthy, but after half an hour of searching, Illya finally founded a collection of dossiers in some of the world's most notable magi, specifically those that von Einzbers kept an eye out for over the years. Among those were Tokiomi Tohsaka, Lord El-Melloi, and, of course, Zouken Matou.
"Here it is!" Illya picked out the dossier. "Let's see if my masters got something useful on him." Illya skimmed over the information; she was a remarkably quick reader, which helped Illya get through books her masters wanted her to read. "Zouken Matou, formerly known as Zouken Makiri, a magus and a consistent constant for the Holy Grail, maintains neutrality and a shaky peace with the von Einzbern family outside of the Holy Grail War. Motives unknown, suspected to be deranged…"
"This does not tell us anything about his abilities; it's pointless scribble."
"Patience, dummy! I'm getting to that part!" Illya shook her head and continued. "Has discovered a way to prolong his life far past mortal limits, has persisted for at least three centuries, and was one of the originators of the Holy Grail War." Illya read out before turning to her companion. "Those must be the worms…His entire body consists of them, and he can break and form them any time he wants. He's not a human anymore, just some form of magecraft with a will."
"It would make him a dangerous opponent indeed. Any word on his weaknesses?"
"Wait, let me look, there's a bit more here…" Illya continued to read. "Claims to be the head of the 'Matou Household,' but it theorized that he is unable to have biological children, a natural side effect of infusing one's body with excessive amounts of magic." The way this dossier was written was highly frustrating, with constant theories and assumptions and very few facts. "Likely formed an alliance with Tokiomi Tohsaka shortly before the Fourth Holy Grail War." That only confirmed what Illya already knew, but she would not turn down an extra bit of information. "Well, that confirms it…" Illya sighed. "Zouken wanted to keep quiet about it, but anyone who can put two and two together would know."
"Know what?" Souichirou wondered, but before Illya could respond, he continued. "Did you say, Tohsaka? As in Rin Tohsaka?"
"I did…Oh yeah, sorry, I forgot to mention that the Tohsaka family is the third out of that trio that designed the war."
"Seems like Fuyuki became a playground for sorcerers without us even realizing it."
"Something like that…But that's not the main point. Here, they suggest that Zouken can't have children, and that thing that I've seen in the basement, I don't have to study it to confirm that it can't have children." Illya sighed. "There has been talk about Sakura being Rin's sister, whom their father, Tokiomi, gave to Zouken for adoption. I don't know if he knew what that geezer would do to her, but either way…This is how it ended up."
"So Sakura and Rin are sisters? And they simply pretend they are not?"
"Yes."
"I see…" Souichirou said, still not displaying any surprise.
"Yeah, I shouldn't have expected anything else from you. Worth a try, though."
"Anything else?"
"Eh…No, I'm afraid not." Illya said with disappointment.
"Then we don't know how to act against him."
"That's not entirely true." Illya jumped down from the ladder. "I hoped there would be some answers, but even if not, we can figure out what to do."
"And that is?"
"Destroy Zouken's body…As is, destroy all the insects that make it up. Perhaps there is a core to be found somewhere inside."
"There isn't much point in an unproven theory."
"Not like we have anything better. We just need to devise some form of pest control…Though I'm not taking into account whatever worms he put inside Sakura…Damn, it's so complicated!" Illya let out with stark annoyance in her voice.
"I expected you to have a clearer strategy in mind if you decided to take on someone like this."
"Sorry for not having years to prepare. Sakura is suffering, and we are the only ones here who can do anything about it before it becomes too late! You're not going complain now after agreeing to help me?"
"I am not complaining, I'm acting in accordance with protocol and how I was trained." Souichirou shook his head. "It is imperative to know as much as possible before the mission, you need to be ready to act in different scenarios and do everything possible to take the target down. You are not in that position."
"Then what do you suggest I do?" Illya said with brimming anger in her voice. This stoicism was quick to drain her out.
"Do as much research as possible, gain allies, and do something that has a chance of succeeding. You are older than you look, but you are still just a teenager. This man is hundreds of years old, even though I am just a child before him. Think of all the experience and strength he accumulated in the world of magic."
"And what about Sakura?! Should I just leave her to deteriorate with this monster? How is that not a betrayal!?"
"How is going on a suicide mission and ruining any chances of actually saving her any better?"
"It just…It would show her that I care and we still have a chance, we…" Illya grew emotional, and eventually, her speech was interrupted by a sudden pain in her chest. "Aaargh!" The girl wailed in pain and fell on her knees, feeling yet another magical circuit weakening and breaking away. Her essence unraveled even further, making Illya feel sharp bursts of pain. Was this just a coincidence, or did her emotions actually influence her body? Has it truly fallen that far to be dependent on simple feelings?
"What's wrong?" Souichirou asked with calm concern. "You have blood coming from your nose."
Even 'better', then.
"I…Damn it, I guess I can't even hold this mess back." Illya clenched her teeth and spoke with shame.
"What 'mess'?"
"I…" The girl took a deep breath, realizing that there was no way around it. "Remember how I told you about the homunculi? Our lifespans are limited because of all the magical experiments they perform, but mine…Is more limited than usual."
"What do you mean?"
"This pain and this blood…It's because magic circuits within me are gradually withering away. They hold a lot of power and mana, but they can only do it for so long…I never intended to have any purpose after the war; I won't live long. I didn't have those aches a few months ago, and now they're getting worse every time…"
"How long do you have?"
"In a few months, I probably won't be able to walk, and in a couple more, I'll be gone…My time is limited, Souichirou, and I will die very soon."
"No matter what you do…" Kuzuki's voice grew quieter. "It was decided for you a long time ago. I understand…"
"No sympathy, eh? Guess that's how you trained assassins operate." Despite her complaining, Illya was glad that Kuzuki did not pity her. Taiga would've treated her like a child and broke down right in front of her. She would have talked about how sorry she was and how certain she was that there was a way to 'cure' Illya. Souichirou's emotional stuntedness allowed him to simply listen and take the information in.
"I'm sorry, Illyasviel." Kuzuki said calmly with a small glimmer of genuine kindness that Illya could swear she noticed, but it was quickly gone from his stern voice.
"Thank you…Sakura is going through the same pain I went through. Even if she doesn't die at the end, her life would probably be limited, like mine. Maybe it already is…I don't want to think about it, I don't!" She couldn't hold back her frustration. "She doesn't deserve to become like me, lonely and spiteful, killing people just because that's the only way I can feel alive…I have no purpose and only a few months to live. I want to do something that's not worthless!" Illya's eyes swelled with tears as she saw a pale figure in her vision, a beautiful smiling woman, so close and yet so far away… "I want to do something my mother would be proud of. Something to stop this cycle that those monstrous mage families started! That's why…That's why I won't change my decision." She stood up and said firmly, leaving Souichirou in silence for a few moments.
"I see…I will stay true to my promise."
"Thanks…"
"But we still need more before we challenge this Zouken, especially with you in such a condition. If only Caster was here…Or any servant worth their legend." Souichirou clenched his fist, but this gave Illya idea which made the girl look up and smile, despite all the pain she was experiencing.
"There is one still around…" The girl said.
"Truly?"
"Yes, I saw him myself some time ago. He is bound to Ryuudou temple, but if we somehow allow him to leave that area…"
"Bound to Ryuudou temple?" Souichirou said in disbelief. "You are not talking about the assassin, are you?"
"Yes, the Samurai in violet robes. You've seen him?"
"Not after the war, no."
"What do you mean, not after the war? Did you have Caster fight him before?"
"No, not exactly. He was our servant."
"Your…Servant?"
Just by the time Illya thought she figured Kuzuki out, he had dropped yet another surprise on her. It was time to ask him questions and learn more about the one who could become the key to saving Sakura.