Ever since Riezel first met Kotetsu and recalled Yoriichi Type Zero, he had been interested in it.
Yoriichi Type Zero, as a combat mechanical doll far surpassing ordinary human capabilities, though ultimately unable to match Yoriichi's prowess, still possessed considerable prowess in this world, not to be underestimated.
In the original work, Muichirou, a Hashira, used Yoriichi Type Zero for training. Despite its significant wear and damage, it could still hold its own against him, proving its capabilities were not inferior to a Hashira.
Surprisingly, such a mechanical doll was actually created solely by a swordsmith without any extraordinary or supernatural powers.
What did this mean?
It meant that with the mechanical doll's blueprint and sufficient technical skills, such a mechanical doll could potentially be mass-produced—truly a frightening prospect!
A mechanical doll equivalent to Level 4, if hundreds or even thousands of them were assembled into an army, might eliminate the need for the Demon Slayer Corps, as demons would have no chance of dominating the world anymore.
Even Muzan would likely retreat if faced with an army composed of Hashira-level mechanical dolls, unwilling to engage with such opponents.
A combat power at Level 4 was considered elite even within Loki Familia, let alone among the knightly orders of the Frenzel family, where out of eight hundred members, no more than a hundred would reach this level.
Moreover, if the conditions were met, these mechanical dolls could be mass-produced in the tens of thousands, even millions.
It was inevitable that Riezel would be interested in this prospect.
Of course, while he entertained the thought, if it were so easy, there would not be just one Yoriichi Type Zero in this world after hundreds of years have passed since the Sengoku period.
Yoriichi Type Zero was the masterpiece of Kotetsu's ancestor, unique in its kind. Even if this ancestor were reborn, it was uncertain whether he could recreate such a powerful mechanical doll again, let alone anyone else.
Kotetsu's family, inheriting the technology from their ancestor, had only been able to maintain and repair Yoriichi Type Zero. No one had ever succeeded in creating another mechanical doll of this caliber, illustrating how advanced the technology used in its creation was, and how difficult it was to master and replicate such a technology.
In such a situation, even if one had the blueprint and the necessary technical skills in hand, creating such a mechanical doll would be extremely difficult.
However, Riezel didn't need to fully comprehend the technical aspects since, after all, he was a magician.
If he could forge swords using structure system magic, couldn't he also create a mechanical doll using it?
After all, this mechanical doll didn't rely on magic metal, didn't employ any extraordinary means, and wasn't an alchemy item, magic item, or special equipment—it was merely a doll with a sophisticated design concept and mechanical technology.
Riezel could perfectly imprint the design into his mind, memorizing every part and material, construction, and placement of the mechanical doll, and then replicate it using magic, mimicking it exactly.
Since it was akin to copy-pasting, he didn't need to read the text or understand its meaning—one click and it was copied down.
Even if he couldn't understand what was written in it, the content was completely consistent with the original version. When others saw it, the information they received and accepted would be the same, and the effect it had on both sides was no different.
In this way, Riezel could mass-produce mechanical dolls equivalent to Level 4, effectively carrying a Level 4 army wherever he went.
Riezel couldn't help but feel chills down his spine as he thought about the idea.
And so, after done pondering, Riezel turned to Kotetsu.
"Your family must have a blueprint for Yoriichi Type Zero, right? Something like its design drawing or concept?"
"Blueprint?"
As Kotetsu nervously glanced at the fierce battle between Tanjirou and Yoriichi Type Zero in the clearing, he paused slightly upon hearing Riezel's question, then furrowed his brow uncertainly.
"Something like that should exist... My father used to occasionally look at a book while repairing Yoriichi Type Zero... I think that book might contain this blueprint thingy."
Kotetsu had flipped through that book many times himself, but whether it actually contained the so-called blueprint, he didn't know since he couldn't make sense of it. Still, his father had mentioned that the book contained records related to the mechanical doll left by their ancestor.
After his father's death, he often reread the book, intending to learn everything about the mechanical doll it detailed, but he still couldn't understand it.
It was also because of this that he confirmed he lacked the talent for swordsmithing, let alone for mechanical dolls, leaving him in despair.
"Can I take a look at that book?"
Riezel asked without knowing Kotetsu's thoughts.
"...Alright."
Kotetsu hesitated for a while, then sighed resignedly.
"It's useless to me anyway. Keeping it would only lead to the end of our legacy."
After saying this, Kotetsu reluctantly glanced at Yoriichi Type Zero battling fiercely with Tanjirou, then turned and walked away.
Riezel didn't follow, continuing to watch the battle.
"Water Breathing - Eighth Form - Waterfall Basin!"
At this moment, Tanjirou shouted loudly, descending from above and slashing downward with his black Nichirin Sword, causing a surging water pillar to appear around him, a magnificent display.
However, Yoriichi Type Zero firmly blocked this formidable slash with its two arms holding two swords, deflecting it, while its other four arms wielded four swords to viciously slash, cutting through and dispersing the water pillar.
As a result, Tanjirou was sent flying, rolling several times on the ground before crashing into a tree and barely stopping his tumbling.
"Strong... Very strong..."
Tanjirou gasped heavily, his body covered in dirt and disheveled.
Clearly, faced with Yoriichi Type Zero, which could rival a Hashira but lacked any emotion as a mechanical doll, it didn't show any mercy.
Gripping its six swords, Yoriichi Type Zero charged at Tanjirou again, forcing him to defend himself desperately.
For a while, the scene was filled with rolling water tides, while the constant clash of swords echoed in the air.
Riezel frowned slightly as he watched Tanjirou's desperate and battered appearance.
'It seems like Water Breathing isn't quite suited for him...'
Seeing Tanjirou's movement, Riezel noted the problem.
He could tell that when Tanjirou used Water Breathing, not only did he fail to exhibit the breathing style's characteristic flexibility and fluidity, but he also appeared stiff with every move.
Water Breathing was supposed to be the most stable and durable breathing style for prolonged combat among all breathing styles. Yet, despite Tanjirou having used Water Breathing for such a short time, he already seemed out of breath.
Evidently, this wasn't just due to exhaustion from fending off his opponent's fierce attacks, but also because his breathing couldn't keep up with the gentle rhythm required by Water Breathing.
From this, it was clear that Tanjirou was never a swordsman skilled in the gentle and adaptable ways of Water Breathing but a swordsman burning with ambition, hard work, and determination—a temperament truly unsuitable for practicing Water Breathing.
Tanjirou's Nichirin Sword also attested to this fact.
Its blade, black like Riezel's Hermit, told others that Water Breathing wasn't the most suitable breathing style for him.
'If he used Sun Breathing (Hinokami Kagura) passed down through his family for generations, wouldn't that change the course of the battle?'
Riezel speculated as he watched Tanjirou's Nichirin Sword.
Currently, Tanjirou possessed combat power close to the limit of Level 3. If he switched to Sun Breathing, he should be able to match Yoriichi Type Zero in a short time.
Unfortunately, Tanjirou couldn't.
'Yeah, his body is too weak...'
Why didn't Sun Breathing continue within the Demon Slayer Corps, instead spawning fundamental breathing styles like Flame, Water, Wind, Stone, and Thunder? Of course, the reason lay in Sun Breathing's demanding requirements for the practitioner's body.
Even the swordsmen of the first generation breathing styles couldn't meet these rigorous demands. Tanjirou, with his frail body, was far from capable of wielding the full power of Sun Breathing.
Riezel even suspected that the early death and chronic illness of Tanjirou's father might have been due to forcibly using Sun Breathing without the necessary body constitution, leading to his illness.
Riezel himself didn't face such troubles.
As a first-class adventurer, his body constitution might even surpass that of Yoriichi, leaving no doubt about his capability to use Sun Breathing.
However, Tanjirou was different.
He was a completely ordinary young man with a weak body constitution. If he were to use Sun Breathing, he would likely collapse in intense pain within moments, unable to move.
Seeing Tanjirou on the brink of collapse, Riezel prepared to intervene immediately, but just as he was considering his next move, Tanjirou's breathing suddenly changed rhythm.
"Total Concentration - Flame Breathing..."
In an instant, the water flowing on Tanjirou's sword abruptly transformed into raging flames.
"Fifth Form - Flame Tiger!"
Tanjirou yelled, slashing at Yoriichi Type Zero with a gigantic slash resembling a burning fierce tiger.
*BOOM!*
Amidst the thunderous roar, the flame tiger roared and collided with Yoriichi Type Zero, causing explosions and shockwaves that sent dust and debris flying everywhere.
Unexpectedly, Yoriichi Type Zero was pushed back by this fierce blow, smoke billowing from its entire body as it crashed into a tree.
'Flame Breathing?'
Riezel's foot halted mid-step.
He looked in surprise at Tanjirou, who was readjusting his breathing to calm the rapid inhales.
'So, Rengoku taught Flame Breathing to Tanjirou, huh?'
Riezel was somewhat taken aback.
Although not Sun Breathing, Flame Breathing was clearly the closest breathing style to it.
Such a breathing style undoubtedly suited Tanjirou better than Water Breathing, especially with his burning ambition yet inability to freely use Sun Breathing.
As a result, Tanjirou's movements changed, becoming more aggressive and explosive.
Flames roared, fire waves swept, and with his sword ablaze, Tanjirou relentlessly attacked Yoriichi Type Zero, momentarily locking in a stalemate.
After observing for a while, Riezel heard Kotetsu's voice.
"I'm back!"
Kotetsu hurried back, clutching an old book in his arms.
"Here, this is a book that's been passed down through my family for generations."
Kotetsu handed the old book to Riezel.
"Thank you."
Riezel thanked Kotetsu and immediately started flipping through the pages. Instantly, complex, obscure texts and intricate drawings filled his vision.