"Naruto, there's no way to place a Flying Thunder God Jutsu Rune on the big iron gate in the Sealed Space, even though it looks like a physical object."
It was already late at night, and Minato Namikaze stood beside Kurama's claw, explaining things to Naruto.
After leaving Shikamaru with a message, Naruto had gone straight home. There had been no point in staying there.
Besides, judging by what Shikamaru had said earlier, he would be coming to Naruto often in the future because Lord Hokage still needed him to report on the situation. That made it even more impossible for Naruto to practice the Flying Thunder God over there.
After returning home and eating lunch, he had spent all his time in the seal studying his father's signature technique, the Flying Thunder God Technique.
Even now, Naruto was still pondering the principles behind the Flying Thunder God Technique.
The Flying Thunder God Technique was Space-Time Ninjutsu.
In the ninja world, Space-Time Ninjutsu was actually quite common. The Summoning Technique was one example.
Space-Time Ninjutsu like the Summoning Technique was utterly unreasonable. Once a ninja formed a blood contract with a Summoning Beast, they could summon each other directly, ignoring the boundaries of planes and space.
Nagato of the Shinobi World could even summon the Ten-Tails' husk from within the moon's core directly to his side.
Naruto had seen plenty of scenes like that when he watched Naruto in his previous life.
For example, early in the anime, the toads of Mount Myōboku could be summoned to Konoha by Jiraiya and Naruto. Later, Naruto could also be reverse-summoned by the toads to Mount Myōboku.
Then there were the two storage scrolls Naruto currently carried. Such scrolls also fell under the application of Space-Time Ninjutsu, and they were among its simplest uses.
As long as a ninja carried such a scroll and supplied it with Chakra, they could summon out ninja tools stored in advance, then seal them back inside afterward.
Konoha's Tenten was the most representative example.
Of course, Naruto believed the strongest Summoning Technique was undoubtedly the Impure World Reincarnation.
This forbidden technique, developed by the Second Hokage, could even break through the boundary between life and death, exploit loopholes in the laws of reincarnation, and directly summon the souls of the dead into the present world.
The Flying Thunder God leaned more toward an application of reverse summoning within Space-Time Ninjutsu. However, blood was not the key during the summoning process. What it required was Chakra, and the contracted target was oneself.
The entire process involved using one's own Chakra to sense the coordinate jutsu rune, then summoning oneself to the marked location.
After studying the Summoning Technique, the Second Hokage had specifically developed a unique summoning formula for the Flying Thunder God. During its execution, it also incorporated the Body Flicker Technique and Fuinjutsu.
The first and most important requirement was that the Flying Thunder God Technique demanded an extremely high level of sensory ability.
The main reason Naruto's father's three personal guards had been unable to master the Flying Thunder God was their poor sensory ability.
Separated from one another, the three could not precisely lock onto the jutsu rune marks they had made through their own senses. They could only use the clumsy method of linking their senses to the same frequency, barely managing to perform the Flying Thunder God Formation through cooperation.
Thus, another prerequisite for using the technique was placing jutsu rune marks within the area one intended to move through when activating the Flying Thunder God.
More accurately, the marks had to be placed in advance.
The mark used for the Flying Thunder God was generated from one's own Chakra. At the instant it was left behind, Fuinjutsu had to be used simultaneously to fully complete the mark's formula.
The coordinate jutsu rune left behind in this way would never disappear. During the final Shinobi World War in the Naruto anime, Minato Namikaze had specifically emphasized this to Obito Uchiha.
However, after Obito became the Ten-Tails' Jinchuriki, he reconstructed his body and destroyed the Flying Thunder God Jutsu Rune that had been sealed onto him.
Based on the memories Naruto's father had passed on to him, Minato had received no guidance when he first began studying the Flying Thunder God. He had wasted some time on sensing and understanding Jutsu Runes.
Still, his talent had been exceptional. With the help of his mother, Kushina of the Uzumaki Clan, he had quickly gotten started with the Flying Thunder God by studying the Uzumaki Clan's many unique Fuinjutsu techniques and their distinctive methods of sensory training.
According to his father, Naruto now possessed a massive Chakra advantage, had mastered sensory ability, knew Fuinjutsu, and had also been taught the core summoning formula of the Flying Thunder God.
What he needed to do now was simple. First, place a jutsu rune mark at the location he wanted to move to. Then sense the Chakra within the mark he had left behind. As long as his Chakra resonated with the Chakra inside the mark, the special reverse-summoning formula within it could instantly summon him over.
After feeling that he met the conditions to begin learning the Flying Thunder God, Naruto had originally intended to challenge his father's Data Puppet in the Mirrored World. He wanted to experience what it felt like to face the Flying Thunder God first.
Instead, he found that he could not challenge Minato Namikaze's Data Puppet. The prompt said there was insufficient data on the relevant person.
After that, when he challenged other Puppets in the Mirrored World, he wanted to try placing jutsu rune marks on them. He planned to see what it would be like to use the Flying Thunder God directly in battle.
But whenever he used Shadow Clones to gang up on a Puppet and tried to place a mark, the battle would inexplicably end by force.
The notification sounded as if it had been poisoned, constantly spamming: Please complete the relevant data.
After discovering that he could not practice the Flying Thunder God in the Mirrored World, Naruto turned his attention back to the Sealed Space.
Within the seal, Naruto had tried it out. Ninja tools such as Kunai could be projected inside, and every other ninjutsu he knew could also be used in the Sealed Space.
Then he set his sights on the sealing gate.
He tested it. The gate could be attacked with ninjutsu, and he could even scrape sparks off it with a Kunai.
Yet he simply could not place a Flying Thunder God Jutsu Rune on it.
That was why Minato Namikaze had explained at the beginning that the iron gate was not actually a physical object, but merely something materialized by the seal.
Looks like I'll have to find an opportunity to avoid the surveillance around me. After thinking that, Naruto decided to put aside the matter of practicing the Flying Thunder God for the time being.
After pondering for a while, Naruto took two steps toward Minato Namikaze beside the iron gate and asked, "Father, shall we study how to add nature transformation to the Rasengan now?"
An awkward look flashed across Minato Namikaze's face. "Uh, Naruto, while I was alive, I was always researching how to add nature transformation to the Rasengan. I even specifically sought guidance from Master Jiraiya."
"But Master Jiraiya, who had also learned the Rasengan, hadn't managed it either. Later, we considered whether we could force a breakthrough with the enhancement Senjutsu gave Chakra, but by the time I learned Senjutsu, I had already become Hokage. I had very limited time to visit Mount Myōboku."
"You know what happened afterward. I simply never had the chance to study in depth how to add nature transformation to the Rasengan."
"So when it comes to adding nature transformation to the Rasengan, you and I are actually starting from the same point."
Naruto's heart sank when he heard that. He knew his father's idea of using Senjutsu was correct.
In the Original Work, although Naruto had painstakingly used the nature transformation of Wind Release Chakra to upgrade the Rasengan into the Rasenshuriken, the Rasenshuriken at that stage was actually only fifty percent complete.
Not only was its state unstable, it had to be used at point-blank range. Its side effects were enormous, harming both the enemy and the user.
Later, after Naruto went to Mount Myōboku and learned Senjutsu, the Rasenshuriken in Sage Mode finally had no flaws at all. It could be thrown perfectly, and only then was the nature transformation considered one hundred percent complete.
Today truly had been one setback after another. At this rate, he would have to reconsider his next training plan as well.
"Heh. Fourth Hokage, you're the one making things so complicated! Why not just strengthen the Rasengan further on its existing foundation? Like the Tailed Beast Bombs we Tailed Beasts fire—the more Chakra there is, the more powerful the Tailed Beast Bomb becomes."
Kurama, who had been lying nearby, suddenly braced himself on a claw and sat up as he spoke.
"Uh... Strengthening the Rasengan further like a Tailed Beast Bomb would require a huge amount of Chakra. My Chakra..." Minato Namikaze froze. He suddenly realized that his son's situation was completely different from his own.
His son possessed the Uzumaki Clan's physique. He did not lack Chakra at all.
Naruto's eyes also lit up. What Kurama said was completely feasible. In the Original Work, the Rasengan had an advanced version called the Big Ball Rasengan after being strengthened from its base form.
Kurama had long since told him how a Tailed Beast Bomb was condensed. Thinking of this, Naruto immediately acted. In the blink of an eye, he condensed a fist-sized Rasengan in his palm, then began gradually pouring more Chakra into its existing form.
Inside the Sealed Space, the Rasengan in Naruto's hand grew larger and larger. Soon, it swelled to the size of a soccer ball, and it looked as though it would continue expanding.
Seeing this, a strange thought flashed through Naruto's mind. He suddenly controlled the Chakra within the oversized Rasengan and began compressing its density according to the method used to condense a Tailed Beast Bomb.
Minato Namikaze noticed something was wrong and was about to speak, but Kurama immediately extended a claw, signaling him not to interrupt.
The oversized Rasengan in Naruto's hand rapidly shrank again. After several breaths, he had compressed the entire Rasengan back down to the size of a fist. Then it turned black.
Through Naruto's senses, the Chakra density inside the thing in his hand—whether it could still be considered a Rasengan or not—had been compressed to an unimaginable degree. Several rings of irregularly rotating halos surrounded the pitch-black sphere.
Those halos resembled the planetary rings in science fiction films from his previous life, bands of matter endlessly revolving around a planet.
"What kind of Rasengan is that?"
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