A challenge letter. A duel of swordsmanship.
Seeing the letter, Mifune's interest was instantly piqued. In his decades as a samurai, the only true life-or-death battle he'd fought with all his strength was against Hanzo. Unfortunately, while Hanzo's swordsmanship was excellent, he was more ninja than samurai, and had coated his weapon with poison.
So, in that battle, Mifune had been defeated by Hanzo's sword, but also by his poison.
In the many years since, his skills had improved dramatically, and the Land of Iron revered him as a Sword Saint. Yet, he was never able to challenge Hanzo to another duel to prove the true measure of his swordsmanship, a fact he considered a great regret.
The next morning—
The sky had just begun to lighten, before the first rays of sun touched his courtyard, when Mifune arose. He bathed, lit incense, and ate breakfast an hour ahead of schedule, eating only until he was sixty percent full to avoid any discomfort during the battle.
Kneeling in his courtyard, he quietly wiped his katana with water and a soft cloth, but he couldn't contain the excitement bubbling in his heart.
Someone who would send a formal, open challenge was certainly no ninja. The visitor was surely coming to test their sword skills. Mifune cherished the few samurai who remained, a group now as rare as pandas. When samurai came to him for guidance, he never held back, teaching what he could.
Unfortunately, with ninja running rampant, most talented individuals chose that path. Those who made a name for themselves in the ninja world with a sword were more often ninja than samurai.
People like the White Fang of Konoha, Hanzo, and later, Killer Bee, who nearly cut Sasuke in two.
Breathing in the fresh morning air, feeling the gentle breeze, Mifune adjusted his condition to its absolute peak, awaiting the arrival of his guest.
Soon, the sun had fully risen, and the common folk were beginning their day. A young figure with black hair, black eyes, and a Nichirin Blade in hand slowly walked into Mifune's residence.
Mifune had left his main gate open long ago to welcome his challenger. He sat quietly in the courtyard, waiting.
"You've finally arrived. It's quite a surprise. I never expected you to be so powerful, and so young. May I ask your name?"
Mifune was taken aback when he saw Uchiha Nagi. A master this young was a rare sight, especially among samurai.
"You flatter me, Master Mifune. My name is Uchiha Nagi, a survivor of Konoha's massacred Uchiha clan. I'm also the man with a one-hundred-million-ryo bounty on his head at the black market," Uchiha Nagi said with a smile.
He saw no reason to hide who he was. The confidence granted by the Flying Thunder God was just that great. Besides, there were probably very few people who could actually claim his head for that bounty.
"Heh heh. You needn't worry in the Land of Iron, Your Excellency. There are only samurai here, no ninja. And the Land of Iron will forever remain neutral. No nation would dare send a large force of ninja to invade this place."
Uchiha Nagi smiled and nodded. "Then, Master Mifune, shall we begin? Since I first learned the sword, I have never had the guidance of a true master. You are revered as the Sword Saint. I am certain I will benefit greatly from this match."
Mifune laughed heartily. "The benefits are mutual. When helping a fellow swordsman improve, one also improves oneself."
With that, Mifune slowly stood up. He wasn't a particularly tall man, appearing like an ordinary man in his fifties. As he walked to the center of the courtyard, he exuded no discernible aura, seeming no different from a common old man.
Uchiha Nagi took a deep breath, gripping his Nichirin Blade. Mifune's sword was not yet drawn, but Nagi could already feel the sharp edge of its intent pressing down on him.
Clang!
With the sound of a blade being drawn, their swords met. The clash of metal rang out, sending sparks flying.
Mifune turned, and in a reverse motion, his sword moved as if returning to its sheath, thrusting unstoppably toward Uchiha Nagi.
Tsubame Gaeshi!
Startled, Uchiha Nagi quickly used his Water Breathing, creating a twisting whirlpool to redirect and evade Mifune's strike.
Pfft!
Mifune's sword pierced clean through a large tree in the courtyard, yet the surrounding area was undamaged. His control over his blade was so masterful that, apart from the point of impact, he caused no harm to the rest of the tree.
"Seventh Form: Drop Ripple Thrust!"
Uchiha Nagi countered, his sword striking swiftly toward Mifune. Mifune used an unorthodox technique, bringing his sword up from below in a C-shaped curve. Uchiha Nagi was forced to alter his own form.
"Seventh Form: Drop Ripple Thrust—Curve!"
Both were the Seventh Form, but the latter was adapted to handle sudden changes and counter any unexpected attack.
Seeing this, Mifune's eyes lit up in admiration. "Excellent! Excellent swordsmanship! In all my years of traveling the ninja world, I have never seen such a technique!"
"You're too kind, Master Mifune. Let us continue."
"Gladly."
"Ninth Form: Splashing Water Flow..."
"Splashing Water Flow—Turbulent..."
"Tenth Form: Constant Flux..."
Even the strongest form of Water Breathing was parried by Mifune. In terms of raw power, Mifune was not a top-tier Kage-level fighter. He probably couldn't defeat Might Guy with seven gates open, let alone the Fourth Raikage.
But in terms of pure swordsmanship, he was, without a doubt, the number one in the ninja world.
At least for now. Kakashi's father, after all, was a ninja, not a samurai. Though his swordsmanship was strong, he fought by combining it with ninjutsu.
Seeing his Water Breathing being blocked, and realizing that Mifune seemed to be analyzing and beginning to deconstruct it, Uchiha Nagi switched to other sword styles.
The speed of Wind Breathing...
The explosive power of Thunder Breathing...
The overwhelming force of Flame Breathing...
Finally, Uchiha Nagi switched to the most powerful style: Sun Breathing. His body had mostly recovered from the battle with Guy. As long as he used Sun Breathing for a short time, the strain wasn't too great. The multiple injections of White Zetsu's bone marrow were making his body stronger by the day, even without training.
"Sun Breathing, First Form: Waltz!"
For the first time, Mifune was a half-step too slow. A cut appeared on his clothes.
"Magnificent swordsmanship! This is the strongest style I have ever seen. Young friend, please, unleash the subsequent forms. Don't worry if you accidentally injure me, or even kill me."
Mifune was starved for knowledge of the following forms. He spoke as if his own death was of no consequence, a profound desire to understand this new path filling him. It felt as if he could learn this truth in the morning, he could die content that evening.
"Very well. Be careful, Master Mifune."
"Sun Breathing, Second Form: Blue Heaven!"
"Sun Breathing, Third Form: Raging Sun!"
"Sun Breathing, Fourth Form: Parhelion Rainbow!"
"..."
"Twelfth Form: Flame Dance!"
CLANG!
Finally, as the twelfth form of Sun Breathing was unleashed, the unexpected happened. Amidst a shower of sparks, Uchiha Nagi's Nichirin Blade snapped in two. Mifune's sword, however, was completely unscathed—a true masterpiece of a blade.
"Hah... I lose," Uchiha Nagi said, sighing as he looked at his broken sword. It had finally reached the end of its life, though it had been a fine blade.
Mifune also sheathed his sword. His body was covered in bloody cuts. Their duel had been a contest of pure swordsmanship. Though Mifune had been wounded multiple times by Nagi's Sun Breathing, the injuries were not serious—all flesh wounds that hadn't struck any vital points.
"In this match, it is actually I who have lost."
Mifune was cheerful, not angry in the slightest.
"Let us speak as equals. As I am older, I will call you my young brother. If you don't mind, you can just call me your old brother."
"Of course I don't mind, Brother Mifune."
"Hahahaha, Brother Nagi!"
"Come, please, inside. Let's talk over tea."
A servant soon brought tea. She wanted to bandage Mifune's wounds, but he waved her away.
After taking a sip of the fragrant tea, Mifune set down his cup with a satisfied expression.
"Brother Nagi, the Sun Breathing you just used... I feel like it's... missing something."
Mifune frowned in thought. "Unlike the other sword styles you used, this Sun Breathing seems to have a final form. This form should serve as the synthesis for the entire style. Once unleashed, its power should be equivalent to the previous twelve forms combined, perhaps a hundred times stronger."
Uchiha Nagi was filled with admiration. As expected of a samurai who dedicated his life to the sword. He had been right to seek him out for a spar. While Mifune couldn't win against the famous Kage-level fighters in terms of raw power, when it came to the analysis and instruction of swordsmanship, he was currently the strongest in the ninja world.
Uchiha Nagi put down his tea. "You are correct, Brother Mifune. Sun Breathing has a Thirteenth Form. This thirteenth sword requires the continuous execution of the first through twelfth forms in a single, unbroken cycle that repeats itself. That is the Thirteenth Form."
But that was just a theory. In reality, only the creator of Sun Breathing, Yoriichi Tsugikuni, could do it. The idea of "continuous use" was a bit of a joke; only by achieving a speed great enough to execute all twelve forms in the time it takes to perform a single strike could it be called the ultimate form.
However, how could one unleash the variations of thirteen different sword forms in a single strike? It wasn't as simple as just swinging the sword fast enough for thirteen moves to come out.
In short, the Thirteenth Form could only be understood, not described. Tanjiro himself didn't know it, so Uchiha Nagi had nowhere to copy or reference it from.
"Is that so? What a pity. Brother Nagi, if you have no urgent matters, you are welcome to stay here for a while. Perhaps we can develop this Thirteenth Form together."
Anyone else might have thought Mifune was coveting his sword style, but Uchiha Nagi didn't mind. It was just the forms. Without the corresponding Breathing Method, even if one learned Sun Breathing, they couldn't unleash much of its power.
"Alright. I was thinking the same thing. I'll impose on your hospitality for a few days then, Brother Mifune."
Mifune laughed heartily. "No imposition at all! I have also refined my own swordsmanship thanks to you. By the way, your sword is broken?"
"Yes. It's been with me through many battles. It was a good sword, but it can no longer keep up with my strength. I've been looking for a swordsmith who can forge me a new blade."
Uchiha Nagi then took out his meteorite and showed it to him, explaining his difficulty in finding a suitable smith.
Mifune frowned, and after a moment, he said, "Perhaps I can help you find a swordsmith. But I can't be sure if he'll be up to the task."