"So this is the main body."
Shirou examined the brain worm writhing in his hand.
The little thing was oddly… kind of cute.
"Guh… kill me!"
Despite having no vocal organs, the brain worm let out a shrill, almost comically dramatic cry, like a defeated knight.
"I refuse."
At a glance, Shirou saw through Zouken Matou's lie, as well as the secret behind his immortality.
"Just as I thought. The moment you're killed, you transfer into another worm as a vessel… so that's the kind of spell you set up. Hmph. I'm not falling for that."
Reflected in his amber eyes was the old Magus's grotesque, fragile true form.
Even Zouken Matou wavered at the sight of what he had become.
Shirou could feel the emotional fluctuations coming from the worm in his hand, but he paid them no mind, speaking in a calm, even tone as he slowly drove him into despair.
"You really went all out just to stay alive. That mind of yours could've been used for something worthwhile, yet you wasted it on this.
Don't worry. As I am now, I can't truly kill you. But I'm not going to let you escape either.
There's still a lot I want to ask you. About the mysteries of this world. You're going to tell me everything."
As he spoke, Shirou glanced at Sakura Matou, who stood nearby with her shoulders drawn in.
"But before that… I need to deal with this pest properly.
Look. Your cute granddaughter is already scared of what her grandpa looks like now."
Still smiling, Shirou toyed with the brain worm in his fingers like a mischievous child.
"What?"
"Didn't you want to 'live'? Then I'll grant that wish."
"Y-you brat… what are you planning? What are you going to do to me!?"
Zouken Matou felt a chill seep into his very soul, and struggled even more frantically.
At that moment, Shirou Amamiya showed a bright, innocent smile, like some gentle being out of a fairy tale.
"I'm going to help this ugly old man become someone his grandchildren will love."
As he spoke, Shirou took out something utterly absurd.
If the brain worm had eyes, they would have bulged out like a fly's.
"This is the new body I prepared for you. Cute, right?"
What appeared in Shirou's hand was Shinji Matou's favorite bath toy.
A little yellow rubber duck.
"No—!"
Zouken Matou writhed desperately, but the boy held him firmly in place.
At the same time, Shirou began to perform Magecraft.
Transferring a soul attached to a brain worm into another object and fixing it there was, under normal circumstances, an extremely advanced spell.
Even for the genius witch of Britain it would be trivial, but for Shirou, a third-rate Magus, it should have been impossible.
Fortunately, this old worm had already modified himself thoroughly to allow repeated reincarnation and survival.
In the most literal sense, he had woven the cocoon that trapped him.
Although he still couldn't manipulate the soul itself, transferring only the consciousness without preserving the soul was something even Shirou could accomplish.
"Aaaaaaaaah!!"
Zouken Matou felt himself being forcibly pulled out of the worm's body, and let out a panicked scream.
He truly longed for eternal life.
But in the end, that desire was to live on as a living being, not as an object.
And this… this was nothing more than a bath toy.
If he had been attached to something with Mystery, like a grimoire, he might at least have retained his sanity.
But this was just an ordinary rubber duck.
No history. No legacy. No Mystery.
Just his grandson's toy.
Zouken Matou would lose himself completely. If this was the price of eternity, he would rather be beaten to death with a slipper.
"S-stop… You can't do this…"
Zouken Matou wailed in agony.
"You can't… you can't do this…!"
"I'm just sincerely helping you achieve your wish for eternal life, old man."
Shirou looked at him and let out a small sigh.
"No one ever understands this job, no matter where I go. Being a savior is tough."
…We were all deceived.
Myself, Shinji, the Tohsaka girl… all of us were fooled by him!
Gentle? Calm? Someone who hates evil and stands for justice?
That brat is nothing like that at all!!!
Beneath that carefully layered disguise—
Zouken Matou's thoughts, as a living being, came to an abrupt end.
In the next instant, his soul was erased, while his consciousness was forever sealed inside the rubber duck.
From that moment on, Zouken Matou no longer possessed any freedom or independent will.
Only when Shirou chose to use him, by supplying mana to the rubber duck, would he be awakened.
In this form, Zouken Matou's existence was strikingly similar to that of his ally, the head of the Einzbern Family, Old Man Acht.
An artificial intelligence storing five hundred years of Magecraft knowledge from the Renaissance to the modern era, with Zouken Matou's personality—
[Magical Terminal Makiri].
Looking at the rubber duck in his hand, Shirou Amamiya smiled in satisfaction.
Now that he had returned to this world, what did he lack most?
Information.
Knowledge.
An understanding of how this world worked.
During his time in Britannia, aside from learning fragments like the legends of the Knights of the Round Table through drifting records, Shirou knew almost nothing about the Proper Human History of his homeland.
If he had to start from scratch, it would take years.
That wasn't a problem. He was only seven, after all. Time was on his side.
The real issue was that Magecraft in this world treated secrecy as an absolute rule.
Even if he wanted to learn, an orphan like Shirou had no way in.
But now, he had obtained a living archive named Zouken Matou.
The genius Magus Makiri would answer any question he had. It was like skipping five hundred years of effort in one go.
"Sorry, Sakura. I kept you waiting."
Seeing what Shirou had just done, Sakura Matou instinctively felt afraid.
But then she remembered that he had just saved her from the swarm.
Recalling the warmth of his arms around her, her heart softened.
"Shirou-nii-san, thank you for saving me."
"Yeah. Let's head back."
"O-okay."
Sakura Matou hurried after him, stealing a glance at the rubber duck in his hand.
"Ah, sorry. It's your grandfather, after all. It's a bit rude for me to just hold onto him like this. Here, take him."
Shirou tossed the rubber duck toward her.
Sakura flinched aside with a look of disgust, letting "Grandfather" fall to the ground.
"Please don't throw something like that at me, Shirou-nii-san."
"S-something like that!?"
Her icy tone left Shirou visibly deflated.
After escorting Sakura back to her room, Shirou knocked on Kariya Matou's door.
Starting tomorrow, he would be the new head of the Matou family.