The Zero-Tails... there were some discrepancies from what he had imagined.
A person's negative emotions and hatred weren't simply a matter of being more abundant in adults and less so in children. Only by experiencing a tragedy of the deepest darkness could enough negative energy be born to nurture the Zero-Tails to maturity.
And Shinno's method was to find a girl with a high physical compatibility with the Zero-Tails, then care for her, cultivate her, and let her form deep bonds with the village. At the same time, he would foster a master-student, even a father-daughter relationship with her.
The final step was to destroy the village just as she was coming of age, plunging her into darkness. Then, in a final, cruel reversal, he—the elder who had always shown her kindness, who had been her teacher and father figure—would reveal himself to her in the most desperate state imaginable, a side she would never want to see. This would allow the Zero-Tails to fully mature.
Of course, that still wasn't enough. The conditions for its growth had been met, but it still needed to feed. It would be best if a jinchuriki was present, allowing the Zero-Tails to absorb a full meal of chakra at once to become a complete being. The reason Shinno went through all this trouble was that he himself was an adult. The negative energy a person created for themselves wasn't that great, because they already knew the truth.
"I see. One last question. How did you solve the problem of certain death after opening the Gate of Death?" Nagi asked the one thing he most wanted to know.
"I developed a dark ninjutsu myself," Shinno said dazedly. "It allows me to pay the price of death after opening the Gate of Death by sacrificing several decades of my lifespan..."
"So, that's the truth behind your survival? Then every time you open the Gate of Death, your lifespan is nearly exhausted. That's not much different from dying, is it?" Nagi pressed, frowning.
"I have the complete version of the Living Corpse Reincarnation. When my lifespan runs out, I just need to find a compatible body and reincarnate..."
Nagi: "..."
So it's a nesting doll situation. After all that, the truth behind Shinno's immunity to the Gate of Death was just the Living Corpse Reincarnation. Great, another Orochimaru. No, not quite. Orochimaru's jutsu was incomplete; he had to finish it himself. The true, complete version was with Shinno.
"Show me your Living Corpse Reincarnation jutsu." Nagi was incredibly curious about the ninja world's first immortality jutsu. While it certainly had its side effects, to be able to create such a wicked, long-life jutsu through body-snatching was the mark of a genius.
Some might say, wasn't Kakuzu's jutsu also a form of immortality? Not really. Kakuzu's jutsu only stole hearts to maintain his body's youth. But what about the brain? Kakuzu couldn't replace his brain cells, could he? After a person reached adulthood, their brain cells were constantly dying. After a few hundred years, Kakuzu's body might be immortal, but his brain would die from excessive brain cell loss. Unless his jutsu could also replace the brain... So, in reality, the only thing that could be barely called true immortality was a wicked jutsu like the Living Corpse Reincarnation, which replaced the entire body. After all, not even the Sage of Six Paths could achieve physical immortality.
After squeezing every last drop of dark medical ninjutsu knowledge from Shinno, Nagi began to extract the Zero-Tails. As for how to make it a complete being, Nagi hadn't yet figured that out. How many unlucky souls in the ninja world had suffered such extreme tragedy? Nagi thought for a long time but couldn't come up with anyone. Tragedies were common, but finding someone with high compatibility who could become the Zero-Tails' jinchuriki and had experienced enough tragedy to generate the necessary dark emotions... that was a bit difficult.
[It's not hard at all. Give it to me. Am I not your dark side? Seal the Zero-Tails inside me, and I'll fuse with it. Wouldn't it be great to have me as your dedicated nuclear battery, supplying you with power from now on?]
A voice, more familiar than his own, echoed in Nagi's mind.
"Damn it! Why can you speak in my head?" Nagi gritted his teeth.
[I don't know. Maybe I was drawn by the Zero-Tails' immense dark power. Well? Want to give it a try?]
"Get lost!"
[Don't be like that. Remember what White Ichigo said in Bleach? If two beings with the exact same appearance, abilities, and power exist, one becomes the king who dominates the battle, and the other becomes the mount that amplifies their combat power. What's the difference?]
"Not interested. Get lost!"
[It's instinct... The worlds may be different, but many principles are the same... If we become one, our combat power will surely multiply several times over.]
"Hmph."
You think I can't do anything about you?
Nagi picked up a mirror and stared into it with his Mangekyo. The powerful genjutsu began to hypnotize himself. In an instant, it was quiet. Nagi had locked his dark side deep within his soul.
[One day, you will agree.]
"Damn it. I have to find a way to deal with this guy," Nagi muttered, his face grim. He hadn't expected his dark side to be more troublesome than anyone else's. What was the reason? Was it because he had lived two lives? Was his soul different from others'? His inner demon seemed to have seen Bleach, which made it even more certain of its own existence, stubbornly believing it was the White Ichigo of this world. Because they both came from the same soul and shared the same memories, his dark side didn't deny him, nor did it have the same "hate the world" negativity as Dark Naruto. Instead...
Nagi frowned. He seemed to have had an epiphany. His dark side was, in fact, a desire to unleash himself, to unleash the self that had been restrained by morals and laws his entire previous life. In this world where strength reigned supreme, when his dark side was born from the Waterfall of Truth, it felt no hatred, only excitement. Its desires were the same as the ones he'd had as an otaku in his past life: to possess the greatest power in the world, to get with all the attractive female side characters in the ninja world, and then to stomp on Madara and punch Hashirama...
"Damn it, if I really did all that, I'd be a complete madman," Nagi muttered, shaking his head. Fantasies are just fantasies. If I actually did those things, I'd become a great demon lord.
He decided to deal with this dark side problem later. Fortunately, it was just a memory, and it hadn't gained much of his power. Besides making a racket, it couldn't do anything. And the Waterfall of Truth wasn't powerful enough to split a person's soul. In the end, it was Nagi's own problem. As a transmigrator who had grown up in another world, the memories of two worlds had created two personalities within his soul. The other people in the ninja world didn't have this problem. They could just go to the Waterfall of Truth a few more times and eventually eliminate their inner demons. But for Nagi, it was incredibly difficult, a massive headache.
Shaking his head, he pushed the miscellaneous thoughts from his mind. The priority now was to figure out how to get this Sky Fortress, this behemoth, moving. The Zero-Tails didn't have enough energy...
"What an idiot. Did I forget I still have the Three-Tails?" Nagi slapped himself on the forehead. He had finally remembered the Tailed Beast he still hadn't returned to Mei Terumi.
Following the map Shinno had provided, Nagi opened a hidden door in the main control room and arrived at the heart of the Sky Fortress. This was it. The Zero-Tails wasn't here, but the Three-Tails was. He could start it up right away.
Nagi excitedly released the Three-Tails and placed it in the center of the jutsu formula, providing energy to the Sky Fortress.
The ninja and villagers of the Sky Ninja Village were, in reality, just small fry. Without a nation, without a stable territory to provide funds for the ninja, how could they train and improve their strength in peace? By farming and doing business on the side while being a ninja? Or by becoming bounty hunters?
Today, something they would never forget happened. The underground iron shell that had been said to be a secret weapon for years, actually began to fly.
First, the ground shook violently. The people of the Sky Ninja Village thought it was an earthquake and rushed out of their homes in a panic. But then they saw a mountainside slowly rising into the sky. As it rose, the dirt and rocks on its surface began to fall away, finally revealing its true form: a sky castle of immense size.
A villager muttered in disbelief, "Is that... the Sky Fortress?"
"The Sky Fortress?"
"That's right! That's the ultimate war machine left behind by our old Sky Ninja Village, the Sky Fortress! Hahaha!"
"Lord Shinno has succeeded! Long live Lord Shinno!"
One by one, the villagers and Sky Ninja began to cheer, throwing whatever they were holding into the air.
"Long live Lord Shinno!"
"Lord Shinno..."
After a while, the crowd began to feel that something was wrong. Shinno was piloting the Sky Fortress, the war machine of the Sky Ninja Village, and he was leaving without a backward glance...
"He's... gone?"
Whoosh.
A gust of wind blew past. The people of the Sky Ninja Village still couldn't quite believe the fact that Shinno had abandoned them and run off with the Sky Fortress...
"Who are you? How did I get here?"
A young girl's voice pulled Nagi back to reality. He slapped his forehead. He had forgotten about the child on the operating table. The anesthetic had clearly worn off, and she had just woken up.
"What's your name?" Nagi asked.
"My name is... Amaru," the girl said timidly.
There was no mistake. This was the Zero-Tails' jinchuriki, the girl Shinno had taken in as his student, the one whose spirit he had slowly crushed until she had tried to commit suicide by jumping from the Sky Fortress, only to be saved by Prince Naruto.
"How long have you been with Shinno?"
"Th-three months."
Nagi thought for a moment and asked her for the details. It turned out she was an orphan who had met Shinno while wandering. Shinno, posing as a doctor, had been traveling the ninja world offering free medical care. When he met her, he discovered her high compatibility with the Zero-Tails and took her in as his student, bringing her to the Sky Ninja Village...
"From now on, you'll follow me. You can be my student if you like. If you don't want to, I'll let you down right now."
"I understand."
As a wandering orphan, Amaru agreed without hesitation. Perhaps her feelings for Shinno hadn't grown that deep yet.