"Alright. You can speak freely with me now."
After the old man, Yoriichi Tsugikuni, had eaten his fill and strolled through the town for an hour, he finally led Nagi to a small grove and looked at him with a smile.
"Let me guess. You're very eager to learn the Sun Breathing swordsmanship, aren't you?"
Seeing that he had been seen through, Nagi didn't hide it. Defeating Muzan was secondary; his primary reason for using the Edo Tensei on Yoriichi was to learn his divine swordsmanship.
The Sun Breathing that Tanjiro knew was hidden within the Hinokami Kagura, and its power had likely been diluted over generations. He couldn't even complete a full cycle of the Sun Breathing forms until he was on the brink of death, at which point he finally grasped the complete Thirteenth Form. However, at that point, they were facing a Muzan who had been weakened countless times over. He had been injured by a massive explosion and poisoned by Tamayo's multi-stage concoction, leaving him unable to even self-destruct and split apart to escape. And even then, Muzan could still fight several Hashira at once.
Yoriichi, on the other hand, had faced Muzan in his prime and had grievously wounded him with a single technique. His swordsmanship was the pinnacle of perfection; calling him a Sword God was no exaggeration. He had slain one thousand five hundred pieces of the scumbag boss with a single move. So, to learn the most primordial version of Sun Breathing, it was only natural to seek out the man himself.
"That's right, senior. I happened to learn Sun Breathing from a boy in the Demon Slayer Corps, Tanjiro Kamado, but it's not entirely complete. So, I was hoping to take this opportunity to learn the original version from you."
"Tanjiro Kamado... To think he really passed down my Breathing Style. Back then, besides leaving Sun Breathing with the Demon Slayer Corps, I also gave it to my friend, Sumiyoshi. He promised me he would pass it down properly. I never thought he would actually succeed..."
Yoriichi seemed to be lost in reminiscence for a moment. Then, he bent down and picked up a tree branch.
"Heh heh. Come, young man. Show me the swordsmanship you've learned. Let me see how far you've progressed with your Sun Breathing."
"Alright."
Nagi drew his Nichirin Blade. Under the bright daylight, its amplifying effect immediately kicked in. The surrounding light dimmed as if it were an overcast day, and the blade began to burn with the flames of the sun.
"A fine sword. A divine weapon that is truly one of a kind. What is its name? If I had such a blade back then, Muzan would have been slain by a single stroke of my sword," Yoriichi said, his eyes wide with admiration as he looked at the Raikiri.
Nagi rolled his eyes internally. If you hadn't been like a villain who loves to monologue and had just delivered the finishing blow, Muzan would have died long ago. You wouldn't have needed any external help.
"It's called Raikiri."
"Hmm. A good name, but it feels a bit... odd," Yoriichi said, scratching his head. He really couldn't understand why a sun-like Nichirin Blade would have such a strange name.
"Senior, I'm coming," Nagi said, readying his attack.
"Come."
Over the past few days, Nagi had used the laboratory at the Butterfly Mansion to cultivate some of the Hashirama cells. After tempering them with Genya's demon-eating cells, he had injected them into his own veins. The effect was excellent, better than he had imagined. The after-effects of using Sun Breathing in his great battle with the A-B brothers had healed in a single day. His body, which should have taken half a month to recover like Kakashi's, was full of life and vigor in just one day. The chemical reaction between Hashirama and Uchiha cells was more than ten times stronger than that of the White Zetsu cells.
"Sun Breathing, Thirteenth Form..."
Nagi took a deep breath. With the release of Sun Breathing, the Nichirin Blade Raikiri became a sword of the sun. With every swing, it carried long trails of solar fire. Any flowers, grass, or trees it touched or grazed would be scorched and withered, yet the flames did no harm to their master.
A complete Sun Breathing Thirteenth Form, with the dance of the Nichirin Blade, transformed into a small, bright sun and shot towards Yoriichi.
But Yoriichi, who seemed to be a bent-over old man, suddenly looked up. As the attack approached, he was as calm as still water, without a ripple of emotion, like a stone, a great tree.
"Sun Breathing, Thirteenth Form..."
Swish—
The two of them passed by each other.
Nagi stood in place, his clothes riddled with holes.
It was the same Thirteenth Form, a single move from his opponent, but Nagi's frantically spinning Mangekyo was telling him: Cannot copy. Cannot copy...
A profound and mysterious sword technique had deftly struck Nagi's own attack. And after that single move, Nagi felt as if he had been struck by a tree branch over a thousand times all over his body.
One sword transformed into a thousand forms, and a thousand forms became one sword. This was the pinnacle of Yoriichi's swordsmanship. Because he was in a reanimated body, he no longer had to worry about his physical limitations or strength, so this strike was nearly as powerful as when he had been in his youth.
"Ah, what a divine weapon," the old man Yoriichi said, tossing aside the tree branch in his hand with a look of regret. It was already half-burnt; the solar flames of Raikiri had still managed to scorch it.
"I lost. Senior's swordsmanship is truly... the pinnacle of perfection," Nagi said with a bitter smile, sheathing his Nichirin Blade.
If he had fought Yoriichi in his Gundam, he might have had a chance of winning. But in a close-quarters sword fight? He probably needed another ten years of practice.
"You rely too much on those peculiar eyes of yours. Heh heh..." Yoriichi sat down on the grass and patted the spot next to him, gesturing for Nagi to join him.
Nagi didn't stand on ceremony. He sat down and took out two bottles of soda. The old man had seemed to take a liking to this novelty from centuries in his future during their walk.
"You were able to use the Thirteenth Form, so you must have grasped the Transparent World during a battle where you gave it your all. But just now, when you fought me, I couldn't feel it," Yoriichi said, taking a sip of his drink. He continued, "The Transparent World is a name the Demon Slayer Corps came up with. In reality, it is more akin to a state of unity with nature, as the Buddhists and Taoists would say. You must first close those eyes of yours to get used to this state. Do you know how many forms were contained within my single strike just now?"
Nagi shook his head. "I don't know. My eyes couldn't see it clearly at all. It seemed like one strike, but it also seemed like a thousand."
"That's right. Because no matter how strong the dynamic vision of the naked eye is, it has its limits. But a person's perception is endless," Yoriichi said with a smile. "My single strike just now contained one hundred repetitions of the Sun Breathing's Thirteenth Form."
Nagi: "..."
One Thirteenth Form contained twelve moves. So, one hundred times twelve? That was over a thousand strikes, just as he had guessed.
"I understand. I will try to close my Sharingan and fight using the Transparent World," Nagi nodded.
"That's the spirit. Close your eyes and feel that transparent world. If you could grasp it once on the brink of life and death, you can grasp it a second time. First, try to comprehend it. When you're ready, I will practice with you again," Yoriichi said with an encouraging nod. "Only by skillfully using the Transparent World can you stack countless Thirteenth Forms of Sun Breathing into a single strike."
Nagi sat to the side, recalling the scene of his great battle with the A-B brothers. Suddenly, he remembered something. The Sage Art he had gotten from Orochimaru... he had been unable to make any headway with it. But now, wasn't there a master right here? Asking him for help would be perfect.
"By the way, senior, I have something here that is no less than a Breathing Style. It's called Sage Art. But I've been unable to master its cultivation. I was wondering if you could help me take a look."
"Hm? Sage Art? What is that? Let me see," Yoriichi said, his interest piqued by the name. Was there something that could rival a Breathing Style?
Nagi took out the Sage Mode scroll from Ryuchi Cave that he had gotten from Orochimaru, opened it, and began to explain its contents to Yoriichi.
The reason Nagi was doing this was because the Sage Art of Ryuchi Cave was clearly designed for snakes. The high mortality rate for humans who attempted to learn it was only natural. So, finding a knowledgeable person to help modify it into a Sage Art suitable for humans was the right way to go. He wasn't going to be as foolish as Naruto and his son. Naruto had succeeded thanks to his protagonist plot armor, and his son Boruto's success was also due to plot armor. Just let anyone else try. Even Orochimaru had been forced to modify the Sage Art into his Curse Mark. It was clear that this technique was incredibly difficult to master. Even the genius Second Hokage, Tobirama, hadn't learned it.
But Yoriichi, the creator of Sun Breathing, a natural-born prodigy of the Transparent World, clearly had the ability to modify it.
Yoriichi said with great interest, "Interesting. This old man will study it. You go on and comprehend the Transparent World. When this old man has figured it out, I will practice the sword with you again."