Isekai Adventure in the Dimensional Cube

That was an ancient sword, housed in a worn scabbard, which appeared to have a considerably long history.

"Wha-, what's going on?"

Kotetsu, who was screaming, suddenly widened his eyes when he saw Reiger pull an ancient sword from Yoriichi Model Zero's body.

"A sword inside an automaton?"

Tanjirou was equally surprised.

Clearly, nobody had expected that a sword would be hidden inside Yoriichi Model Zero.

Only Reiger acted as if he had always known about the sword's existence. After pulling out the ancient sword, he promptly reattached Yoriichi Model Zero's head as if nothing had happened.

Then, Reiger extended his hand and pressed it against Yoriichi Model Zero's chest, activating magic that made a dazzling array appear on his hand that was touching the automaton.

However, the array flashed only briefly—appearing and disappearing so fast that before Tanjirou and Kotetsu could react, it was already gone.

And Yoriichi Model Zero, at some point, became as pristine as if it had just returned from factory maintenance—intact, conveying a sense of cleanliness and tidiness.

Reiger withdrew his hand.

"There, that should do it," Reiger said, returning to Kotetsu's side. "Go check how well it's been repaired."

"...Okay."

Kotetsu approached with an expressionless face.

His mind simply couldn't keep up with the rapid developments.

First, Reiger had created an automaton overnight that surpassed his ancestors in craftsmanship, then Yoriichi Model Zero turned out to have a sword inside that he knew nothing about, and now Reiger simply pressed Yoriichi Model Zero's chest and claimed it was fixed. Kotetsu's brain had completely crashed from these bewildering events.

Now, he felt that nothing mattered anymore.

Since it was all beyond his comprehension, he might as well act as though he hadn't seen anything.

Only Tanjirou continued to be amazed by Reiger extracting a sword from within Yoriichi Model Zero.

"This sword is...?"

Tanjirou looked puzzled and uncertain.

Reiger didn't immediately clear up Tanjirou's confusion, but instead lifted the ancient sword and drew it from its scabbard.

The next second, a stretch of rusty blade was revealed before Reiger and Tanjirou.

"This is Tsukiguni Yoriichi's sword,"

Reiger, while holding the rust-stained sword, blurted out such a surprising statement.

"Tsukiguni Yoriichi's sword?"

Tanjirou, predictably, was startled.

"There's no need to be so surprised; it's quite reasonable to deduce," Reiger said indifferently to Tanjirou. "Yoriichi Model Zero was created during the Sengoku period, hence this sword was placed inside it back then."

"Although the blade has rusted, one can still feel a faint sunlight energy from the steel, proving it is a Sun Wheel Blade."

"Furthermore, despite the faint traces of rust, the dark hue on the blade is discernible, which undeniably indicates that it is a black blade."

"A black Sun Wheel Blade, existing since the Sengoku period—aside from the famed Tsukiguni Yoriichi, I cannot think of anyone else to whom this sword might belong."

This line of reasoning successfully crashed Tanjirou's mind as well.

However, as the original protagonist, Tanjirou, though visually impaired, keenly noticed the highlight.

"Mr. Tsukiguni Yoriichi's sword was a black blade too?" Tanjirou said, overtaken with surprise. "Aren't black blades only wielded by those who are unskilled at swordsmanship?"

This was a rumor neither too big nor too small within the Demon Slayer Corps.

Since a Sun Wheel Blade could reflect the breathing style that suited its wielder, anyone who received a Sun Wheel Blade would instantly know which direction they should develop in.

But there were two exceptions.

One was those who could not make the Sun Wheel Blade change color.

The Sun Wheel Blade only responded to swordsmen with a certain level of skill; if someone who couldn't even learn swordsmanship well picked it up, then the blade would not change color.

In other words, those who could not make the Sun Wheel Blade change color were universally acknowledged within the Demon Slayer Corps as rookies and duds.

The other exception was those whose Sun Wheel Blade turned black.

A black blade was an extremely rare Sun Wheel Blade, and within the centuries-long history of the Demon Slayer Corps, only a handful of people possessed one.

Yet a black blade failed to correspond to any of the breathing techniques inherited within the corps, leaving the bearers clueless about which direction to take.

Therefore, swordsmen with black blades generally struggled to find a breathing style that fit them, earning them a reputation for lacking skill.

Just like Tanjirou, who had used the ill-fitting Water Breathing in his fight against Yoriichi Model Zero yesterday, resulting in his combat abilities being somewhat diminished and affected.

Over time, a notion took root within the Demon Slayer Corps that swordsmen with black blades could never truly excel—they believed such a blade was only held by those lacking in skill.

Tanjirou obviously had also heard this rumor.

In response, Reiger only wanted to say,

"Indeed, Tsukiguni Yoriichi's Sun Wheel Blade was a black blade, and so is my Sun Wheel Blade," Reiger said with a smile that was not quite a smile. "Are you implying that both Tsukiguni Yoriichi and I are swordsmen with poor skills?"

Tanjirou immediately shook his head wildly without even thinking.

What a joke, if he said that out loud, he was afraid Tanjirou would be drowned in spittle.

Tsukiguni Yoriichi was the founder of the breathing techniques, the greatest swordsman of the Warring States period, and also the most powerful swordsman in the history of the Demon Killing Corps.

Reiger, on the other hand, could easily slay Upper Moon demons and effortlessly crush pillar-level swordsmen, and was regarded as the hope for defeating Kibutsuji Muzan.

If these two were not skilled in their craft, then all the swordsmen in the world would be rookies and trash.

"Although I cannot be certain," Reiger said, "I believe that the person whose Sun Wheel Blade turns black is most suited for the Sun Breathing style." Reiger continued, "Tsukiguni Yoriichi's blade was a black blade, my blade is also black, and even your blade, which has inherited the Sun Breathing, is a black blade. With so much evidence laid out here, the theory that black-bladed swordsmen are most in tune with the Sun Breathing has, at least, an eighty percent chance of being correct."

"And since black-bladed swordsmen are so rare, and the breathing techniques of the Warring States period were not as widespread as they are now, this black blade that has been passed down from that period is very likely the Sun Wheel Blade Tsukiguni Yoriichi used during his lifetime."

Of course, this was what the original story said, and that was why Reiger was so confident in his tone.

"The ancestors of the Little Iron family had a deep connection with Tsukiguni Yoriichi, and since they were able to create the Yoriichi Prototype taking him as a model, it proves they had observed Tsukiguni Yoriichi from a very close distance and done so with great detail."

"For them to acquire the Sun Wheel Blade used by Tsukiguni Yoriichi during his lifetime is not strange at all."

"And when they were making the Yoriichi Prototype, they hid the blade inside the prototype's body."

Reiger shrugged his shoulders, saying so.

"As for what his intention was, that is something we cannot know."

Upon hearing this, Tanjirou turned his gaze to the ancient blade in Reiger's hand.

This ancient blade was covered with rust, making one doubt whether it could cut through a leaf.

However, after Reiger said this blade was the Sun Wheel Blade used by Tsukiguni Yoriichi before his death, Tanjirou, for some reason, suddenly grew fond of it.

He felt as if the blade was waiting for him to find it, and it held an inexplicable attraction to him.

"Ting-a-ling——"

The Sun Wheel Earrings on Tanjirou's ears swayed as well, producing a sound like a wind chime.

After over three hundred years, the two personal items that once belonged to Tsukiguni Yoriichi reunited at this moment.

Reiger then held the ancient blade, looked at it for a while, and then activated a magic spell.

"Hum!"

With a humming sound, Magic Power flowed into the ancient blade, causing the rust on the blade to disappear bit by bit.

Before long, the ancient blade was no longer rusty but revealed its true sharp edge, as if it had been re-polished, becoming brand new and bright.

Reiger repaired Tsukiguni Yoriichi's blade, sheathed it, and tossed it to Tanjirou.

"Eh?"

Tanjirou was initially startled, then clumsily caught the blade.

"Here," Reiger said, "You've found something useful for you, haven't you?"

This statement reminded Tanjirou of what Reiger had said before when he saw the Yoriichi Prototype yesterday.

Yesterday, Reiger had mentioned that they might find something useful for Tanjirou here.

Was that something this blade?

"Yes, it is this blade," Reiger said as if seeing through Tanjirou's thoughts, "The Kamado family inherited the will of Tsukiguni Yoriichi. This blade in your hands is also reaching the person it was meant to reach."

"But you found it, Brother," Tanjirou quickly said, "And Brother, you've also inherited Mr. Tsukiguni Yoriichi's Sun Breathing."

"I am different from you," Reiger said calmly, "The presence of the Sun Breathing in my hands, rather than being an inheritance, is more like a trade and entrustment. The connection between that swordsman of the Warring States period and me cannot compare to that of your family."

"But still…" Tanjirou had hesitation written all over his face.

"I already possess the Recluse, which is much stronger than this ancient blade. Even if you gave it to me, I would just let it collect dust in the warehouse," Reiger said with a slight smile, "Instead of that, it's better for you to use it."

While saying this, Reiger also turned to look in the direction of Little Iron and said, "You don't mind, do you?"

Little Iron glanced in their direction, his words filled with indifference.

"Take it. After all, I have no talent for forging swords, I'm not a blacksmith. Entrusting a blade from the Warring States period to me, I couldn't even polish it properly, so let's consider it compensation for repairing the Yoriichi Prototype."

While saying this, Little Iron continued to touch the shell of the Yoriichi Prototype, checking its condition.

He had largely confirmed that the Yoriichi Prototype had been repaired.

Moreover, it had been repaired perfectly.

At this rate, the Yoriichi Prototype could last at least a few more decades.

Little Iron felt satisfied with this, and thus he no longer cared about the blade.

"Alright then,"

After hearing both Reiger and Little Iron say so, Tanjirou finally nodded and accepted the blade.

Reiger, turning to Array who followed closely behind him, suddenly smiled again.

"I hope this gives those demons a great surprise."