Indeed, this was not the swordsmanship training he had experienced at Kazami's.
Nor was it the high-altitude physical conditioning he had endured under Urokodaki.
This time, Kuguruma Natsunishi faced a simple forge and a heavy iron hammer held firmly in his hand.
All he had to do was strike the iron.
It sounded simple.
Actually doing it, however, was far more difficult than Kuguruma Natsunishi had anticipated.
Matsuyama's requirement was that every swing of the hammer had to be delivered with full strength, all while maintaining the rhythm of the Stone Breathing throughout the entire process.
Simply mimicking the Stone Breathing was already a struggle, and the moment the hammer struck, the resulting vibration would cause his arm to go numb, naturally shattering the rhythm of his breathing.
He turned his head to look at Matsuyama beside him.
The cultivator, working at his own forge, was completely different.
He stood firmly on one leg, gripping the iron block tightly with tongs. The old hammer he held was much larger and more worn than Kuguruma Natsunishi's.
Clang!
The sound was steady and concentrated, as sparks cascaded like a waterfall.
Clang!
His breathing, his heartbeat, and the force of his strike seemed to form a subtle resonance.
Clang!
Kuguruma Natsunishi could even faintly hear the minute sounds of impurities being squeezed out of the iron and the metal becoming dense under the heavy pressure.
"Your breathing faltered again."
Matsuyama didn't even look up, his voice blending with the roar of the bellows.
"Grip the hammer as if you are gripping your breath. Remember, it is not you swinging the hammer; it is the breath swinging the hammer. The earth is using your arm to swing that hammer."
It sounded quite mystical.
Kuguruma Natsunishi had no choice but to grit his teeth and push through, continuing to swing the hammer alongside the retired swordsman.
Clang! Clang! Clang—
His strikes were sharper and more scattered compared to Matsuyama's.
Sure enough, after several hits, his maintained breathing technique collapsed once more.
"Continue."
Matsuyama still didn't look at him, silently contending with the iron ingot before him.
It was as if the one practicing the breathing technique wasn't Kuguruma Natsunishi, but Matsuyama himself.
"To turn a piece of iron into excellent steel, you cannot rely solely on brute force, nor can you rush it."
"Steady your heart."
Kuguruma Natsunishi nodded.
Setting aside his exhaustion, he felt that the game developers had designed this training segment quite interestingly.
Among the many games that chased the thrill of combat, there were very few that allowed players to quietly contemplate the "core of cultivation."
Of course, it was mainly because he couldn't trigger the training acceleration animation right now.
The auxiliary acceleration could only be used for highly repetitive actions that had already been mastered.
When it came to learning brand-new techniques or engaging in fierce battles with demons, he had to experience every second personally.
Since the crow had brought news of his promotion, the sound of the hammer had echoed for days.
By the third day, Kuguruma Natsunishi found the sensation Matsuyama had described.
The number of times his breathing technique broke while striking the iron ingot decreased significantly.
And by the fifth day, when his mind and soul were completely immersed in the resonance of breathing and striking, a familiar system function finally unlocked: the blacksmithing training entered a "skippable" state.
Kuguruma Natsunishi naturally chose to fully commit, allowing every drop of sweat and effort to be hammered into his current cultivation with maximum efficiency.
Time became like visible flowing sand, slipping away rapidly outside of his senses.
Heating, forging, cooling, reheating...
Sweat had long since soaked his clothes, forming salt crystals on his skin. His arms went from aching to numb, and then to a strange sensation of being one with the hammer handle.
He felt the subtle resistance and extension of the iron under the hammer.
The Stone Breathing progress bar, which hadn't even reached the halfway mark, began to grow slowly and steadily in the system panel, like a heartbeat.
On the other side, Matsuyama Daimon silently watched Kuguruma Natsunishi, who was immersed in a state of flow.
This child was more talented than he had imagined.
Practicing multiple breathing techniques was no easy feat; after all, a person has only one set of lungs and one brain.
Mastering a single breathing technique is already a rigorous test of body and will; practicing two or three means memorizing several sets of completely different breathing rhythms and sword forms.
It was an extremely heavy burden.
Not only would it not significantly increase one's strength, but one would also have to switch to the required breathing mode at the right moment.
In his view, other than broadening one's horizons, it had almost no practical meaning.
To truly be able to switch between two or more breathing techniques fluently and use them freely in life-and-death combat against different situations... the difficulty was far higher than drawing a circle with the left hand and a square with the right.
Water Breathing, with its soft and inclusive nature, might allow a talented person to master it alongside others.
But what about Stone Breathing?
It was heavier, more difficult, and placed extremely high demands on the user's temperament and physique.
However...
Matsuyama never doubted whether the boy before him could master Stone Breathing.
He was merely curious about what Kuguruma Natsunishi would realize from these mountain rocks.
And what new path he would forge using the wills of Stone, Water, and Wind.
The will of the mountain and stone lies in endurance.
Mr.Fudo (Fudo Hino) had learned hardness and strength here. Much like that boy's personality, he was like a silent volcano on the outside, but scorching lava surged within.
He had tempered his own breathing from the mountain rocks.
Eventually, he became the "Stone Hashira" of the corps.
Himejima, however, was completely different.
When he first arrived... he carried a sadness heavier than a mountain.
He was compassionate, sensitive, and easily moved to empathy. He seemed soft, but deep within his core lay a hardness and wariness that ordinary people could hardly reach.
It was as if Himejima, in the depths of his heart, actually lacked trust in many people.
But that didn't mean the boy's talent was lacking.
In Matsuyama's humble view, Himejima would surely become even stronger in the future, even surpassing his predecessor, the "Stone Hashira."
He had hoped so much that Fudō and Himejima could stand side-by-side to support the future of the Demon Slayer Corps.
But...
Thinking of the recent obituary regarding Mr.Fudo's death in battle, Matsuyama couldn't help but feel a lingering sorrow.
That boy was only twenty-six; he clearly had a great future ahead of him.
He had died so quietly in the wilderness.
According to the "Kakushi" report, when they found Hino, only a small portion of his body remained in the wasteland.
It was covered in combat wounds and stained with an extremely potent poison.
Clearly, he had perished in a battle against an Upper Rank demon.
Even after exhausting all his strength, he had been unable to defeat the opponent.
At the moment of death, he must have been filled with regret and pain.
Matsuyama had wondered more than once: if he had been by his disciple's side at that time, could he have saved his life?
If they had faced that Upper Rank together...
Perhaps with his own abilities, he still wouldn't have been able to change the outcome.
But even if they couldn't win, perhaps... at least he could have given that boy a sliver of a chance to survive.
The Demon Slayer Corps needed more steadfast and powerful swordsmen.
This was why he had become a cultivator and why he had accepted Kazami Sarayoshi's request.
And the Kuguruma Natsunishi before him...
He allowed Matsuyama to see a possibility entirely different from Hino or Himejima.
Undoubtedly, this child possessed extraordinary talent.
But what intrigued Matsuyama more was that elusive "uncertainty."
Regarding the slaying of demons, Kuguruma Natsunishi possessed a very pure obsession that even confused Matsuyama.
In his eyes, that degree of obsession was far higher than a blood feud.
It was as if, just as humans need to breathe, Kuguruma Natsunishi viewed slaying demons as a matter of course.
It was as if... as if he were born to slay demons.
And regarding cultivation, the boy's behavior was somewhat baffling.
At times he seemed restless, but in the blink of an eye, he could become more focused than anyone else—a level of concentration far exceeding any child Matsuyama had ever seen.
Furthermore, his physical fitness was far beyond what a swordsman at his stage should have.
It was second only to Himejima Gyomei's initial state.
The most surprising thing was the breathing techniques; he actually intended to achieve mastery in all five basic styles.
Yet he was different from other ambitious but shallow swordsmen.
While most swordsmen struggled to master even one style, he had not only mastered Wind and Water.
Now, he was on the verge of learning Stone Breathing as well.
Matsuyama let out a soft breath, a flash of relief and faint worry in his eyes.
Just as Matsuyama's mind was becoming filled with stray thoughts, there was a new development from Kuguruma Natsunishi's side.
The sound of the hammer striking the iron ingot changed completely.
[Stone Breathing]
Activated.
[Stone Breathing]
┣ LV.0 Unpolished Jade
┣ Progress: 1/250
┗ Sword Form Mastery: 0/10