Isekai Adventure in the Dimensional Cube

Reiger had considered this possibility ever since he first saw the little automaton and was reminded of Tsukiguni Yoriichi Model Zero.

As a combat automaton whose performance far exceeded that of humans, even though it ultimately did not possess the ability to match Tsukiguni Yoriichi, its combat prowess was still not to be underestimated in this world.

In the original story, Tokitou Muichirou, one of the Pillars, had trained against Tsukiguni Yoriichi Model Zero. The automaton was almost broken, yet it managed to exchange blows with him to the point where, even in defeat, it proved that its capabilities were no weaker than those outside the rank of Pillars.

And such an automaton was created by a swordsmith who possessed no extraordinary powers.

What did that mean?

It meant that, with blueprints and a certain level of technology, such automatons could potentially be mass-produced!

This was a terrifying prospect.

If automata of Lv.4 capability could be mass-produced—say, a thousand or even eighteen hundred to form an automaton legion—the Demon Slaying Corps might no longer be necessary, and demons would not be able to strut around unchallenged.

Even Kibutsuji Muzan, when faced with an army consisting of Pillar-level automatons, would probably choose to retreat immediately rather than engage with such a force.

The combat strength of Lv.4 was rare even among Loki's kin, the core and elite few. Even in a powerful Marquis family like Franzel's, among the eight hundred knights, it was unlikely that more than a hundred would be at that level.

However, provided the necessary conditions were met and the mechanical puppets could be successfully mass-produced, it wouldn't just be eight hundred—hundreds of thousands, even millions, could be gathered.

Reiger couldn't help but be intrigued by this idea.

Of course, while it was one thing to think about it, if it had been so easy, hundreds of years since the Sengoku period wouldn't have passed with only one Tsukiguni Yoriichi Model Zero existing in the entire world.

Tsukiguni Yoriichi Model Zero was the masterpiece of the little automaton's ancestral swordsmith, and there was only one. Even if that ancestor was to be reborn, it was uncertain whether another automaton of the same strength could be created, let alone by someone else.

The descendants of that ancestor at the little automaton's household had always been able to only maintain and repair Tsukiguni Yoriichi Model Zero, with no one ever managing to create another. This alone demonstrated how advanced the technology to create Tsukiguni Yoriichi Model Zero was, and consequently the difficulty in mastering it.

Given such circumstances, even if one had the blueprints and related craftsmanship, creating an automaton like this would be extremely difficult.

However, Reiger did not need to fully understand the technology.

He was a magician.

If he could use construct magic to forge blades, couldn't he use the same magic to create mechanical puppets?

These mechanical puppets didn't incorporate any magical metals or extraordinary means, and were not alchemical tools, magical devices, or special armaments. They were simply dolls with sophisticated design concepts and mechanical techniques.

Reiger could perfectly memorize the blueprints in his mind, remember every part of the mechanical puppet including materials, structure, and placement, and use magic to reproduce them.

This was akin to copying and pasting. There was no need to understand the content or meaning of the text; one just had to copy it verbatim.

Even if he couldn't comprehend what was written, the content was an exact match to the original. Others would receive and comprehend the information just the same, and the function served by both would be indistinguishable.

In this way, Reiger would be able to mass-produce automata equivalent to Lv.4, essentially carrying a Lv.4 corps everywhere with him, which was a chilling thought.

Consequently, Reiger turned towards Tanjirou.

"Your family should have retained something like the design blueprints of Tsukiguni Yoriichi Model Zero, right?"

Reiger inquired.

"Design blueprints?" Tanjirou was nervously watching the fierce battle in the arena between himself and Tsukiguni Yoriichi Model Zero. Upon hearing Reiger's question, he paused for a moment, then frowned uncertainly, "There should be, I suppose? My father always flipped through a book when repairing Tsukiguni Yoriichi Model Zero; that book might have the design blueprints."

Tanjirou had also flipped through that book numerous times.

He didn't know if it contained design blueprints because he didn't understand them, but his father had mentioned that the book recorded the Mechanical Puppet Technique left by their ancestor.

After his father's death, Tanjirou often perused the book, hoping to learn the Mechanical Puppet Technique recorded within, but he never understood or mastered it.

This was also why Tanjirou confirmed that he had no talent as a swordsmith or in the Mechanical Puppet Technique, which left him rather despondent.

"May I have a look at that book?"

Reiger was unaware of Tanjirou's thoughts and simply made this request.

"...Alright," Tanjirou hesitated for quite a while, then sighed in resignation, "It's useless to me anyway. Keeping it only leads to the end of the legacy."

With that, Tanjirou took one last reluctant look at Tsukiguni Yoriichi Model Zero, still fighting fiercely with Tanjirou in the arena, and then turned to leave.

Reiger did not follow. He continued to watch the battle.

"Water Breathing·Eighth Form·Takuhai!"

At that moment, Tanjirou shouted as he descended from the sky, cutting downwards with his black blade, causing surging water columns to erupt around him. It was a sight to behold.

Yet, his impressive strike was firmly blocked by Tsukiguni Yoriichi Model Zero using the blades in two of its arms in front, while its other four arms, each holding a sword, furiously slashed through the water column, splashing droplets everywhere.

Tanjirou was thrown back, his body rolling clumsily on the ground and only coming to an abrupt stop after crashing into a tree.

"So strong..."

Tanjirou gasped heavily, his appearance disheveled.

Clearly, he was no match for Tsukiguni Yoriichi Model Zero—a foe who could compete with a Pillar.

But as Tsukiguni Yoriichi Model Zero was a mechanical puppet devoid of any emotions and without any intention of holding back, it continued to charge at Tanjirou with six swords in hand, leaving him no choice but to brace himself for the onslaught.

For a moment, waves surged non-stop in the arena, and the clashing of swords echoed endlessly in the ears.

Reiger frowned slightly as he observed Tanjirou's exhausted and clumsy defense.

"It seems," he mused, "that Water Breathing is not suited for Tanjirou."

Indeed, Reiger had noticed this tendency.

He could tell that when Tanjirou used Water Breathing, he was not only unable to demonstrate the technique's unique flexibility and fluidity but instead appeared stiff in every move.

Water Breathing was supposed to have the strongest endurance in battle among all breathing styles, yet Tanjirou had used it for only a short time before he was already panting for breath. This wasn't just because he was overwhelmed by the fierce offense of the opponent; it was more so because the gentle breathing method of Water Breathing couldn't keep up with him.

Tanjirou was naturally not a Swordsman adept at the gentle and variable nature of Water Breathing but rather a youth with a burning ambition, hardworking and persistent—characteristics that indeed made him unsuitable for practicing Water Breathing.

Even Tanjirou's Sun Wheel Blade proved this point.

His black blade, similar to Reiger's Hermit, indicated that Water Breathing was not the best fit for him.

"If he were to use Sun Breathing, to use his family's inherited Dance of the Fire God, surely the tide of the battle would change, right?"

Tanjirou had nearly reached a Lv.3 limit in his combat strength now. If he switched to using Dance of the Fire God and the most powerful Sun Breathing, he should be able to compete with Yoriichi Zero Style in a short period.

Unfortunately, Tanjirou couldn't harness the power of Sun Breathing.

"His physique is too weak," Reiger noted.

Why wasn't Sun Breathing passed down within the Demon Slayer Corps, instead giving rise to foundational breathing styles like Flame, Water, Wind, Rock, and Thunder?

The reason was that Sun Breathing had very demanding physical requirements for its practitioners.

Even the first generation of Swordsman couldn't meet these demanding requirements, let alone Tanjirou. He simply could not unleash the power of Sun Breathing.

Reiger even suspected that the early death of Tanjirou's father, plagued with sickness in bed for years, might also have been because he practiced Sun Breathing but his physique couldn't keep up, leading to weakness and illness.

Reiger himself didn't have such troubles; as a first-level adventurer, his physique was possibly stronger than Yoriichi's, so he wouldn't have the issue of being unable to utilize Sun Breathing.

But Tanjirou was different. He was just an ordinary young man with a weak physique. If he were to use Sun Breathing, he'd likely collapse in severe pain before long, unable to move a muscle.

Seeing that Tanjirou was about to be overwhelmed, Reiger immediately prepared to step in.

Just as Reiger was considering this, Tanjirou's breathing rhythm suddenly changed.

"Full Concentration·Flame Breathing———"

The rippling streams of water on the black blade in Tanjirou's hand abruptly transformed into fierce, burning flames.

"Fifth Form·Flaming Tiger!" Tanjirou shouted, unleashing a massive slash akin to a burning tiger at Yoriichi Zero Style.

"Boom!"

In the roaring blast, the Flaming Tiger collided with Yoriichi Zero Style, igniting an explosion and an impact that caused a whirlwind of dust.

Yoriichi Zero Style was actually repelled by this ferocious strike, smoking all over, and collided with a tree.

"Flame Breathing?"

Reiger's steps, about to move forward, suddenly halted.

He watched, astonished, as Tanjirou readjusted his breathing and calmed himself down before he finally understood, moments later.

"I see, so Rengoku taught Tanjirou Flame Breathing, did he?"

This was interesting.

Although it wasn't Sun Breathing, Flame Breathing was the closest to it.

For Tanjirou, whose heart burned with ambition yet who couldn't freely use Sun Breathing, this was undoubtedly more suitable than Water Breathing for him to manifest his full potential.

Consequently, Tanjirou's movements changed, becoming more aggressive and explosive.

Flames rolled, fire waves swept through, and Tanjirou, with his black blade ablaze, persisted in his relentless attacks on Yoriichi Zero Style. For a time, he actually held his own against him.

After observing for a moment, Reiger then heard the voice of Kotetsu.

"I—I'm back!"

Kotetsu ran back, holding an ancient manuscript in his arms.

"Here, this is the book that has been passed down in my family."

Kotetsu handed the old book to Reiger.

"Thank you."

Reiger expressed his gratitude, then immediately began to flip through the book.

Sections of complicated, obscure, and difficult-to-understand text and drawings instantly caught Reiger's attention.