"Breathing technique?"
Zoro froze for a moment but didn't refuse.
Since Nami and the others were already asleep, Ren decided to go a bit farther away with Zoro.
They stopped in a valley strewn with broken rocks.
There, Ren began explaining the method of Thunder Breathing that existed within his mind.
Breathing techniques—the greatest reliance of the Demon Slayer Corps in Demon Slayer: Kimetsu no Yaiba—were special training methods capable of strengthening ordinary humans beyond normal limits.
They allowed people to fight day and night without exhaustion, demolish houses in seconds, and even chase after a moving train.
Even in the One Piece world, a breathing method like that could elevate an average person to the level of a Marine Headquarters officer.
Of course, going beyond that would be impossible—the upper limit was fixed.
Right now, Ren was demonstrating and transmitting to Zoro the breathing art that had been injected into his mind as a skill.
Thunder Breathing!
It used controlled breathing to regulate and strengthen the heart and lungs, allowing the bloodstream to flood with oxygen in an instant.
The oxygen-rich blood would then circulate faster through the body, granting the user a temporary surge of power that far surpassed human limits.
As the user's mastery deepened, their body would gradually grow stronger as well.
Yet, where there is gain, there is loss. Among all breathing techniques, Thunder Breathing was the most violent—sacrificing everything for sheer lightning speed.
That didn't mean its practitioners were physically frail—it referred to the burden placed upon the body.
Surpassing one's natural limits inevitably caused internal damage that couldn't easily heal, which was why the art required a certain physical talent.
Ren's normal attributes were unremarkable, but his templates were not.
Whether it was the First Calabash Brother or the Third Calabash Brother template, both could withstand the strain of Thunder Breathing. Combined with his Sword Heart talent, Ren was especially suited for it.
So when Ren began his demonstration, he could immediately feel the power of Thunder Breathing.
However, the process wasn't as smooth as expected.
The Thunder Breathing he possessed had been inserted into his mind as a skill—but it was one his brain remembered while his body did not.
A rhythmic pulse began to influence his body from his consciousness.
Ren unconsciously followed that rhythm, his mind sinking deeper and deeper.
The Sword Heart in his attribute panel began to glow—it was activating its innate potential.
Then,
Ren fully immersed himself in the rhythm—at first it was a heartbeat, then a trickling stream, and finally a wild pulse galloping across the wilderness, echoing like thunder in his head.
"Thunder… the Breath of Thunder… the Rhythm of Lightning!"
Ren forgot the outside world entirely, diving completely into the storming rhythm.
His breathing changed in an instant, as if synchronized with the pulse of lightning itself.
It began rough and uneven, each inhalation and exhalation like a pounding heart.
Gradually, the rhythm slowed, transforming into a flowing river, speeding up as his mastery deepened—
Until it became a violent storm of breath aligned with the cadence of thunder itself.
An indescribable strain seized him, as if a child were trying to swing a blade taller than himself—pouring in every ounce of strength just to maintain control.
Every muscle in his body came alive, twitching as though tiny insects were wriggling beneath the skin.
Each tremor pulsed to the same rhythm as lightning.
Ren refused to yield, gripping tightly to this fleeting moment of enlightenment.
A gust of white vapor burst from between his teeth—like a dragon preparing to roar.
"This is… a breathing technique!?" Zoro's eyes sharpened, and he instinctively took a step back.
The next instant—
Ren's crimson eyes snapped open. Golden streaks coiled around his fingers as his hand seized the hilt of the Nichirin Blade.
The blade within the sheath shimmered instantly into a radiant blue, matching the crackling aura surrounding Ren's body.
Click!
Ren's thumb pushed the blade slightly from its sheath—the blue light trembled like thunder.
"Thunder Breathing, First Form: Thunderclap and Flash!"
A sudden peal of thunder split the air.
A streak of blue light trailed a flaming arc, slicing through a boulder ten meters away.
The blade carved straight down from the top—splitting the massive stone cleanly in half, like a hot knife through butter.
Boom—!
The halves toppled to the ground. Ren closed his eyes, concentrating on recalling every detail of that strike.
Without a doubt, this was the practical application of Thunder Breathing after achieving initiation.
The art was built upon one's physical foundation—it unified every ounce of the user's strength into a single "rope" under rhythmic breathing.
That "rope" became the power source behind the sword techniques of the breathing style.
That was why he could cleave the giant stone with ease—using no more effort than a normal swing.
In his Third Calabash Brother form, Ren would've needed at least three punches to crush that same rock.
If that boulder had been an enemy, much of his strength would've been wasted.
This was the true power of swordsmanship.
After a short rest, Ren opened his crimson eyes and gazed at the blue-hued blade.
A Nichirin Blade reflected its master's spirit—the blue glow was his color.
"Let's call it Raichi—the Thunder Pond."
Ren decided, sheathing the sword and turning to Zoro.
"Well? How does it feel?"
"It's a strange technique," Zoro mused. "Different from what I've learned. You put a lot of focus on breathing and inner circulation."
He looked thoughtful now, realizing this Breathing Technique wasn't something Ren had made up, but a real, systematically integrated secret art.
However—
Zoro frowned slightly. "It's just… too strange. Can a Devil Fruit power really do something like this? Create a whole new art out of nothing—something that even generations of swordsmen might never perfect?
Or did you… steal it from somewhere in the world using your Devil Fruit ability?"
"In this world, you won't find another person who knows Thunder Breathing," Ren said with a smile. "You don't have to worry about plagiarism."
"Tell me what you think of it, though. I want your honest evaluation."
"Fine, if you insist."
Zoro thought for a moment, then said, "It seems like a technique created to help people dig deeper into their potential—mainly for the weak.
Its goal is to let the user channel all their power into a short burst,to exceed their usual strength for a brief moment—and defeat someone stronger."
"In theory, that's exactly right."
Ren recalled the Demon Slayers' fierce battles against demons. Indeed, the breathing styles were born from the desire to let the weak fight the strong.
But that made him curious.
"By the way, how is your sword style different from this? Could you explain?"
From Zoro's words, it seemed his own swordsmanship differed fundamentally from Thunder Breathing—not just in technique, but in principle.
Otherwise, Zoro wouldn't have said 'it feels like something meant for the weak.'
After all, he was not the kind of man to look down on the weak.
(End of Chapter)