Six months slipped by in the blink of an eye.

Kuina, the daughter of Kōshirō, was born safe and sound just a month ago.

As her birth approached, the elusive elder of Shimotsuki Village—Shimotsuki Kōzaburō, who had stayed away from the public eye for more than two years—finally showed himself to Tachibana Kyūjō for the first time.

Since it was their first meeting—and honestly, way too hard to explain—Kyūjō chose not to bring up his request to have a sword forged.

Instead, he focused on showing respect, keeping a good impression, and caring for the old man like a doting grandson.

Once he found out that Kōzaburō loved fishing, Kyūjō immediately seized the opportunity.

He shared every bit of fishing knowledge he had accumulated across countless lifetimes—though, truth be told, most of it came from failed attempts and tragic tales of empty nets.

Still, through persistence and unwavering effort, he eventually unlocked a hidden "mission."

The reward?

A lighthearted sentence from the elder, spoken with a warm chuckle:

"Kyūjō... not bad. If you're free, come fishing with this old man sometime, will ya?"

— — —

That day, baby Kuina cried loudly in Kōshirō's arms.

Kyūjō sighed softly, stepped forward, and held out both hands.

With just a few gentle movements, the baby's wailing subsided. Her tiny face softened, a faint look of contentment emerging, and she slowly drifted off to sleep in his arms.

Kōshirō and his wife watched in quiet awe.

You couldn't really blame them—this was their first child. They had no experience, no mentors to guide them in caring for a newborn.

Kyūjō, on the other hand, was strangely skilled.

In a previous life, he had helped Kagaya Ubuyashiki raise his five children.

And in one surreal arc of his journey through the world of Kimetsu no Yaiba, he had even lived through decades of peaceful family life—growing old, raising kids of his own, all within a dream after his fierce battle against Kokushibō.

Now, as he gazed at the peacefully sleeping Kuina, another face came to mind.

A boy who would one day become a legend.

Zoro—the green-haired, three-sword-wielding warrior, wielder of the "Nama Nama no Mi" Devil Fruit, and future World's Greatest Swordsman. (Not to mention... the king of absurdly named techniques.)

Kyūjō let out a small smile.

"That kid... he should be born soon too."

— — —

1503 of the Sea Calendar (Three Years Later)

This was the year Monkey D. Luffy was born.

On a small, nameless island near Shimotsuki Village, two figures stood facing each other.

Kōshirō, the head instructor of the Isshin-ryū sword style, looked up with quiet resignation.

Standing across from him was his disciple, Tachibana Kyūjō—now fifteen years old and towering at two full meters, tall enough that anyone would have to crane their neck just to meet his gaze.

"How did you even get this tall...?" Kōshirō muttered in his mind.

Over the past few years, Kyūjō had grown at an unbelievable pace. Not just in size, but in strength—far beyond what anyone could call "normal." Even his own teacher now had to look up to speak to him.

Kōshirō was proud of his disciple's progress in every aspect—except one.

He felt Kyūjō still hadn't fully accepted the spirit of Isshin-ryū.

Not the techniques, not the form—but the very foundation of the style: the heart, and the unwavering faith in its ideal of the "One-Hearted Blade."

And so, Kōshirō brought him to this isolated island.

He wanted to show Kyūjō what Isshin Kendō was truly about.

— — —

Kyūjō let out a long sigh.

He didn't really understand why his master had been so eager to fight him lately.

Their training duels had become more frequent—and more destructive.

Maybe Shimotsuki Village was just too small to contain Kōshirō's true ambition?

Honestly, Kyūjō would've rather used the time to refine his Haki.

He didn't even need to fight to understand an opponent's swordsmanship. Just standing before them, his mind could run thousands of possible battle simulations in seconds.

Of course, Kōshirō had no idea about this.

The one flaw in Kyūjō's Infinite Simulation was that the visions only lasted for two minutes. That was the limit of his Endless Sight.

Kyūjō slowly drew his blade from his hip—a long katana crafted in the traditional style of the Demon Slayer world, but built larger to match his towering frame.

This was no ordinary weapon.

It was the personal work of Kōzaburō himself—the legendary swordsmith who had forged it based on Kyūjō's own design a year prior.

Though technically classified as a Ryō Wazamono, its weight and potential couldn't be underestimated.

When handing it over, Kōzaburō had said:

"The strongest blade in the world... is the one forged by the soul of its wielder."

"Kyūjō, I believe in you. Use your Haki—your will—to shape this sword into a true Kokutō. When that day comes, you'll be the strongest swordsman on the seas!"

(At this point in history, Dracule Mihawk had yet to claim the title of World's Greatest Swordsman.)

Kyūjō named the sword Raijinryū—Thunder Dragon.

And he never once doubted that one day, it would become a true Black Blade.

Every sword had the potential to evolve into one.

By imbuing them with Armament Haki, swordsmen could temporarily blacken their blades.

But a true Kokutō—a Black Blade—was something forged through constant use, years of battle, and unwavering willpower.

Once it turned black permanently, it became a symbol of eternal conviction.

— — —

The battle began.

On one side: Wadō Ichimonji, one of the twenty-one Ō Wazamono swords.

On the other: Raijinryū, a Ryō Wazamono with limitless potential.

And this time, Kōshirō held nothing back.

He would show his student the true strength of a sword guided by a single heart.

— — —

Kyūjō stood still.

Though his master hadn't moved an inch, he could already feel the pressure—the subtle power that blurred the lines between man and nature.

Just like five years ago...

Kōshirō had become one with the world.

The air, the earth, the wind—they flowed through him. Within the boundless awareness of Kyūjō's Infinite System, he couldn't find a single opening.

But this time... he didn't feel afraid.

Because now, Kyūjō had found his own path.

— — —

Kōshirō didn't strike first.

He spoke, his voice calm and firm.

"What is Isshin Kendō?"

"The human body has limits. But the human heart... does not. If our hearts are boundless, then our strength can surpass our physical limits."

"To fight limitation with the unlimited... how could that possibly lose?"

Kyūjō paused at those words.

He didn't reject idealism. He knew that belief and determination could awaken extraordinary strength.

But that wasn't the way of his sword.

He believed in the real, tangible strength of the human body.

Like someone chanting, "I am the best" a hundred times in front of a mirror... until they believed it. Until that belief changed their life.

In this world—this chaotic sea of supernatural forces—the shift could be massive.

That was what Haki was, wasn't it?

The power of will, spirit, body, soul, battle instinct—everything unseen, condensed into something real.

If someone could convince themselves deeply enough... that power would erupt.

— — —

But Kyūjō had also inherited the knowledge of a modern world.

He had witnessed firsthand the boundless potential of the human body.

To him, physical strength didn't have a ceiling.

It just needed to be drawn out, refined, and pushed beyond the edge.

So he rejected idealism.

He placed his trust in the body itself.

What he didn't realize...

Was that his absolute faith in himself—

Was a form of idealism too.

— — —

A fun note: both Kozuki Oden and Zoro wielded the same sword—Enma.

But Oden stood at 3.82 meters, while Zoro's just 1.81.

So... does Haki actually stretch the sword too?

That mystery... still remains.

— — —

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