"Naruto, you cannot mark the seal of the Flying Thunder God on the great iron gate of the seal space. It looks like matter, but it is not truly real."
It was deep into the night. Minato Namikaze stood beside Kurama's colossal claws, explaining patiently.
After leaving Shikamaru with only a single line, Naruto had returned home. There was no reason to linger. And from what Shikamaru said, he would be visiting often to carry back reports to the Hokage. That meant training the Flying Thunder God near that place was impossible.
So, after lunch, Naruto immersed himself in the seal space, studying his father's signature technique. From daylight until now he had been pondering its principles.
The Flying Thunder God was a space–time ninjutsu.
And in the shinobi world, space–time arts were not rare. Summoning techniques were the most common example.
Summoning ignored logic entirely. With a blood contract, ninja and beast could tear through dimensions to call one another. Nagato could even rip the husk of the Ten-Tails from within the moon's core and summon it to his side.
Naruto remembered countless scenes from his past life, watching the anime. Gamabunta summoned to Konoha. Naruto himself reverse-summoned to Mount Myōboku. Storage scrolls too simple, but still space–time arts. Input chakra, and weapons sealed within would appear. Tenten was the living example.
And among them all, the greatest space–time forbidden art was surely Edo Tensei. That monstrous technique of the Second Hokage pierced even the veil of life and death, summoning souls from beyond the cycle.
The Flying Thunder God leaned closer to reverse summoning. But instead of blood as a key, it used one's own chakra, with the contract object being the caster himself. The user's chakra resonated with the marked formula, and the technique dragged him across space in an instant.
The seal was permanent. Even Obito, after becoming the Ten-Tails' jinchūriki, had to remake his entire body to shake off Minato's formula.
Minato had learned it with no guidance, fumbling through the runes and sensing. Only by Kushina's help, with her clan's profound sealing knowledge, had he truly mastered it.
Now Naruto possessed what even his father once lacked: an ocean of chakra, sharp perception, mastery of Uzumaki sealing. All the conditions were met.
But when he tried in the Mirror World, things went wrong. He could not challenge his father's data avatar, the message read incomplete data. And when he marked training dummies mid-battle, the system forced the fight to end, spitting out only one phrase again and again: Please complete data.
Blocked on all sides, Naruto turned to the seal space itself. Weapons projected inside. Jutsu worked. But the great iron gate resisted completely. His marks would not hold.
That was when Minato explained that the gate was only a construct of the seal, not true matter.
Naruto closed his eyes, thinking. "Then I will need to wait. And choose the right place, far from their eyes."
He set the matter aside for now and asked, "Father, let us study something else. How do we add elemental nature to the Rasengan?"
Minato's face tightened. "Naruto… in life, I tried. I even sought Jiraiya-sensei's help. Neither of us succeeded. Later, I thought of using senjutsu to force it, but as Hokage I could not afford the time at Mount Myōboku. And then… you know what happened."
Naruto frowned. He remembered the original tale. His Wind Release Rasenshuriken had been unstable, self-destructive, only perfected after he learned sage mode.
Again his plans faltered. First Flying Thunder God, now elemental Rasengan.
"Ha! Yellow Flash, you overthink." Kurama suddenly stirred, red eyes glinting. "Why not simply expand the Rasengan? The more chakra poured in, the greater the power. Just as with our Tailed Beast Bombs."
Minato froze. "To enlarge a Rasengan requires far more chakra than I ever had…" Then his eyes widened. Of course. His son was Uzumaki. A bottomless sea of chakra.
Naruto's gaze sharpened. Yes. There was precedent. The Ōdama Rasengan. But why stop there? Kurama had already shown him how to compress chakra into a Tailed Beast Bomb.
Without hesitation, Naruto raised his palm. A Rasengan bloomed to life, humming with contained force. He poured in more and more chakra. The orb swelled, larger and larger, until it was the size of a football, still growing.
Then a darker thought struck him. His hands shifted. The dense chakra sphere began to shrink, compacting under crushing force, imitating the method of a Tailed Beast Bomb.
"Wait,Naruto" Minato started, but Kurama raised a claw, silencing him.
The sphere shrank back to fist size. But it was no longer blue. It was black.
Naruto's eyes narrowed in wonder. The ball pulsed with terrifying density. Around its surface, faint glowing rings swirled, orbiting without pattern, like planetary rings of some distant world.
"What… What kind of Rasengan is this?"
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