Uchiha Nagi sat to one side, his eyes closed, meditating on the battle from that night, trying to recapture the feeling of being on the cusp of the Transparent World.
Meanwhile, Yoriichi Tsugikuni studied the Sage Art scroll, his brow sometimes furrowed in thought, sometimes breaking into a smile. He could clearly see that the Sage Art of Ryuchi Cave was, in theory, an incredibly powerful technique, but its cultivation method seemed ill-suited for humans. He was very puzzled. Was this thing not meant for people to practice?
Thanks to his innate grasp of the Transparent World, Yoriichi quickly gained control over his reanimated body. And so, he began to experiment with the Sage Art Nagi had given him.
The results were just as he expected. When the Sage Art training failed, his body underwent a strange transformation. Scales began to appear on the surface of his skin, and in a short while, he had become a half-human, half-scaled monster, his pupils turning reptilian.
Fortunately, the regenerative ability of the Edo Tensei body was no joke. Once Yoriichi stopped cultivating natural energy, the residual energy in his body was slowly expelled, and the Edo Tensei reassembled his body, returning him to normal.
Nagi, on the other hand, was slowly recalling the feeling from that night. He soon got up, intending to ask Yoriichi for another lesson in swordsmanship.
"Huh?! Senior Yoriichi? You?" Just as Nagi stood up, he saw the monstrous form of Yoriichi, his face covered in petrified scales, his eyes like a snake's.
"It is nothing. A small matter. I will return to normal shortly. This technique of yours, this Edo Tensei, is truly remarkable. The body of the revived dead can be destroyed and still reassemble itself. After we defeat Muzan, you must never pass this jutsu on to anyone else. It is an affront to the natural order..."
"Rest assured, senior. I will never teach it to another soul. I swear it," Nagi said earnestly. The Edo Tensei was a technique he would teach to no one.
"Then I am relieved." Yoriichi nodded with satisfaction. With his innate Transparent World, he could sense every minute change in a person's muscles and blood. When Nagi spoke, he felt no fluctuation in the man's heartbeat or muscles. He was not lying.
"This Sage Art you gave me... I tried it. The risk is indeed very great. And..." Yoriichi frowned, slowly voicing his assessment, "this technique does not seem suited for humans. It appears immensely powerful, but the side effects are enormous. If the three energies are even slightly out of balance, the user will suffer a backlash..."
Sage Art was a combination of spiritual energy, physical energy, and natural energy. The three had to be in perfect harmony. The slightest imbalance would lead to disaster. The Sage Art of Mount Myoboku would turn you to stone if you failed. The Sage Art of Ryuchi Cave, as Boruto's era would later reveal, would cause the user to be eaten by the snake sages, but in reality, it meant turning into a snake-like beast, losing all reason and becoming a feral creature. The side effects of the Shikkotsu Forest's Sage Art were unknown, but they were likely no better; Tsunade had never learned it in her entire life.
But Sage Art had one major advantage: it didn't require chakra. Chakra was a combination of spiritual and physical energy, requiring a "chakra seed." The people of this world possessed spiritual energy, physical energy, and natural energy, giving them the foundation to practice Sage Art, but they lacked the foundation to practice chakra-based ninjutsu. And natural energy was everywhere, so for Yoriichi, the basics of cultivating Sage Art were no problem.
Yoriichi continued, "This Sage Art, it's more complete than the Mark. It has a very perfect cultivation system, and it can be passed down..."
"The Mark?"
"This thing on my head." Yoriichi smiled and pointed to his forehead. "You might not believe this, but back in my day, any swordsman who cultivated a Breathing Style and awakened the Mark would not live past the age of thirty. I was the only one who awakened the Mark and lived to be over eighty years old."
Nagi said, "Senior, are you perhaps suggesting that this Mark is natural energy?"
Yoriichi smiled and nodded. "We always called it the Mark, not natural energy. Because I was born with the Transparent World, I could perceive every muscle, every blood vessel in my body, and know if anything was even slightly amiss. But others could not."
At this, Yoriichi sighed, looking at the description on the scroll. "It's probably as it's written here: the balance point of the three energies. I didn't understand it at the time, but out of complete control over my own body, without even realizing it, I managed to keep myself alive. But the other swordsmen couldn't do it."
"It was likely this so-called 'natural energy' that was slowly eroding their bodies, which is why none of them lived past thirty. My tragic older brother, to save his own life, sold himself to Muzan..."
"Unfortunately, I spent my entire life trying to find a way for them to live after awakening the Mark, but I couldn't. If I had seen this scroll earlier, I might have been able to save them before I died..."
Nagi asked curiously, "How did you awaken your Mark?"
Yoriichi smiled. "I was born with the Mark. In other words, as it is recorded in this scroll, I have a 'Natural Sage Body'."
"..."
A Natural Sage Body... in the ninja world, there was only one, that fellow Hashirama Senju. But Yoriichi had another cheat: the Transparent World, an innate ability to see through the essence of all things.
Good grief. Are all the Children of Destiny such cheaters?
Unfortunately, Yoriichi was only half a Child of Destiny. His wife and child were killed by a demon, and after defeating the great villain, the scumbag boss Muzan, he let him escape, only to live out his days alone.
Yoriichi began to slowly recount the story of his childhood. His Mark was more like something suited for human cultivation. He had been born with it, and only after a few years did he slowly become aware of its function. He possessed a perception of the world that others did not, and the Mark on his head had made him incredibly strong without him even realizing it. At the age of eight, he could defeat his brother's adult sword instructor with a wooden sword. His young older brother, upon seeing this, had planted a seed of jealousy in his heart.
"With this complete method of cultivating Sage Art, once I have studied it thoroughly, I believe it won't be long before I can modify it into a Sage Art suitable for humans."
Yoriichi sighed. "To invent a Sage Art and write it down as a cultivation system... it might not even be possible in several hundred years. If I'm not mistaken, you didn't obtain this Sage Art from a human..."
Nagi nodded. "That's right. This cultivation system was originally not meant for humans, but for snakes. The survival rate for humans who practice it is infinitely close to zero. That's why I thought, while I have summoned you to the world of the living, to see if you could help me improve it."
"Give this old man a few days to study it. I don't think it will be a big problem. Now, how is your comprehension coming along?"
Nagi nodded. "I've grasped a little. Senior Yoriichi, shall we go again?"
"Yes."
And so, Nagi began his training anew. With the help of the Sword God, Yoriichi, his mastery of Sun Breathing improved by leaps and bounds. The Hashirama cells were constantly enhancing his physique, and unlike before, he no longer needed to rest for over ten days like Kakashi every time he used Sun Breathing in a battle.
The day of Ubuyashiki's premonition was drawing closer. Of all the Hashira, besides the Stone Hashira, only Shinobu Kocho sensed that something was amiss, as Ubuyashiki had brought in Tamayo to help her develop a poison.
Shinobu's hatred for demons was ever-present, but for the greater good, she followed her master's instructions and collaborated with Tamayo. They developed a compound poison. The two most potent components were one that would prevent Muzan from splitting, so he couldn't self-destruct and escape, and another that would accelerate his aging by several hundred times. This was why Muzan later lost nine thousand years of his lifespan and his hair turned white.
One day, Shinobu was talking with her younger sister. The topic of conversation was that, should they encounter the Upper-Rank demon who had killed their older sister, Shinobu would sacrifice herself, feeding herself to him.
"Th-that's unacceptable!" Kanao Tsuyuri was shocked. Shinobu planned to use herself as bait to feed an Upper-Rank demon.
"It is absolutely acceptable. That man has an abnormal obsession with women's bodies, especially strong women. He will not let one go; he will choose to eat her," Shinobu said firmly. "His strength is immense. Even two or three Hashira might not be his match. If I am defeated, and he chooses to eat me, that will be your chance."
"Then let me do it! I'll be the bait!" Kanao said emotionally, wanting to die in place of the sister who was all she had left.
"It's too late. To turn oneself into a wisteria poison bomb requires years of daily injections of wisteria venom. So, the only one who can carry out this plan is me."
"But... but..." Kanao was frantic, but she didn't know how to stop her.
It had to be said, when a woman sought revenge, she was truly ruthless—not just to others, but even more so to herself. To destroy Muzan, Tamayo had turned herself into a poison and fed herself to him. And for the sake of her sister's revenge, Shinobu was also turning herself into a poison to feed Doma.
"I disagree."
A cold voice interrupted them. Uchiha Nagi had walked in. He had been training with Yoriichi for several days and desperately needed to recover. He had come back to the Butterfly Mansion to find Shinobu, hoping to borrow her lab again to clone some more Hashirama cells. He hadn't expected to overhear their conversation. Shinobu was planning to feed herself to Doma? That Upper-Rank Two, that scumbag who loved to eat women.
"You can leave first. I have something to say to your sister," Nagi said to Kanao.
Kanao nodded, a hint of hope in her eyes. She hoped this man could talk her sister out of doing something so foolish.
After Kanao had left, Nagi spoke seriously to Shinobu. "It's fine to want revenge for your sister. But there's no need to sacrifice yourself."
"But..."
"No 'buts'," Nagi interrupted, taking her hand firmly. "I'll help you get your revenge. Trust me. I have the power to do it now."
Nagi opened his Mangekyo Sharingan and looked at Shinobu. In her world, her sister appeared before her again, gently persuading her not to sacrifice herself.
When she came to, Shinobu couldn't help but cry again. Although she knew the sister in the genjutsu wasn't real, she was still heartbroken. For that incredibly kind sister, for the chance to avenge her, Shinobu was willing to pay any price.
"Thank you... for letting me see my sister again," Shinobu said, wiping away her tears.
"Trust me. My strength is on a completely different level now. I can definitely help you," Nagi said earnestly. When he had first arrived, he had only been a jonin who had just awakened his Mangekyo. After a year of copying numerous ninjutsu and fusing with White Zetsu and Hashirama cells, he was now a Kage-level powerhouse who could go toe-to-toe with A.
"This time, Ubuyashiki-san has already formulated a very thorough plan. As long as... mmmph..."
Before he could finish his sentence, he was once again silenced by a pair of lips. The difference was that this time, she had initiated it.
He blinked, looking at her. Shinobu's eyes were closed, a blush spreading from her neck to her face. As a man, he thought for a moment, then wrapped his arms around her, holding her tighter. Then, one hand slowly slipped under her uniform.
"Mmm~" Shinobu whimpered, pushing him away.
Don't get the wrong idea. Nagi hadn't done anything improper. He had simply placed a mark on her body—a level-two Flying Thunder God formula.
"I... my medicine in the lab isn't ready yet. I have to go," Shinobu said, her head down, her face red as she hurriedly left.
"I'll help you..."
The next morning, Nagi gathered the four young protagonists for some special training.
Zenitsu, spurred by the death of his master, had completely shed his cowardly, dual-personality persona. His senior, out of cowardice and fear of death, had encountered the Upper-Rank One, Kokushibo, and had decisively betrayed them, becoming a demon and even rising to the rank of the new Upper-Rank Six. And the retired former Thunder Hashira, Zenitsu's master, had taken his own life out of guilt, committing seppuku without anyone to assist him in the final act. When no one is there to deliver the final blow, the agony of seppuku can last for a very long time. After learning of this, Zenitsu had suffered an unprecedented blow. After crying all night, he had undergone a complete transformation.
"I've called the four of you here today because I have something to teach you," Nagi said, turning to Genya. "First, you, Genya Shinazugawa. I'm going to give you two types of cells. We'll see which one is more suitable for improving your strength."
The two types of cells Nagi planned to give Genya were, of course, White Zetsu cells and Hashirama cells. He believed that this boy, with his ability to eat demons, had the highest potential in the world of Demon Slayer. Unfortunately, he lacked the right kind of cells, which was why in the final battle, he had to keep fighting while looking for demon cells to consume. It was a terrifying ability. If he didn't die prematurely, he could become the most powerful person in the world.