"Naruto, you can't mark the Flying Thunder God formula on the giant iron gate inside the seal space," Namikaze Minato explained, standing beside Kurama's massive paw.
"Even though it looks solid, it isn't a real physical object."
It was already deep into the night.
After parting with Nara Shikamaru earlier and leaving him with a single cryptic sentence, Naruto Uzumaki had returned home. There was no point staying out longer—especially after learning that Shikamaru would be reporting regularly to the Third Hokage, Hiruzen Sarutobi. That forest was no longer suitable for secret training.
Since noon, Naruto had been shut inside the seal space with his father, immersing himself in the legendary technique known as the Flying Thunder God Technique (Hiraishin no Jutsu).
Even now, he was still dissecting its underlying principles.
The Flying Thunder God Technique was a form of space–time ninjutsu.
In the shinobi world, space–time techniques weren't particularly rare. Summoning Jutsu, for example, was one of the most common.
Summoning techniques operated without conventional logic—once a blood contract was formed, a ninja could instantly summon their contracted beast across any distance, even across dimensions.
In the original timeline, Nagato had once summoned the Demonic Statue of the Outer Path, the husk of the Ten-Tails, directly from the moon.
Naruto remembered all of this clearly from the anime of his previous life.
Early on, both Jiraiya and Naruto could summon the giant toads of Mount Myōboku to Konoha. Later, Naruto himself could even be reverse-summoned back to Mount Myōboku.
The storage scrolls Naruto carried now were also a rudimentary form of space–time ninjutsu. As long as chakra was supplied, pre-sealed weapons could be summoned and stored again at will.
Tenten was the textbook example of mastering this art.
But in Naruto's opinion, the most terrifying summoning technique of all was undoubtedly Edo Tensei.
Created by the Second Hokage, Tobirama Senju, this forbidden jutsu ignored the laws of life and death, forcibly dragging the souls of the dead back into the living world.
Compared to that, the Flying Thunder God Technique was closer to a reverse summoning.
Instead of summoning another being, the user summoned themselves.
The process involved using one's chakra to lock onto a previously marked formula seal, then instantly transporting the body to that location.
After studying conventional summoning, Tobirama had developed a unique formula for Flying Thunder God. When activated, it combined elements of Body Flicker Technique, sealing arts, and ultra-high-level spatial perception.
The most critical requirement was extraordinary sensory ability.
Minato's three personal bodyguards had failed to master the technique for this exact reason. Individually, they couldn't accurately sense the formula seals they'd placed.
As a result, they had to synchronize their chakra and activate a simplified group version—the Flying Thunder God Formation.
Another prerequisite was simple but absolute:
Before teleporting, the user had to mark a destination with a chakra-based formula seal.
At the moment the seal was placed, a sealing technique had to be activated simultaneously to permanently anchor the coordinate.
If done correctly, the seal would never fade.
Minato had once personally explained this during the Fourth Great Ninja War, when he told Uchiha Obito that Flying Thunder God marks were permanent.
Only when Obito became the Ten-Tails' Jinchūriki and regenerated his body did the original seal disappear—destroyed along with the flesh it had been carved into.
From the memories passed down by his father, Naruto learned that Minato had struggled immensely in his early days mastering the technique—especially without guidance.
It was only with Kushina Uzumaki's help—drawing on the Uzumaki clan's deep knowledge of sealing techniques and sensory methods—that Minato had eventually succeeded.
Now, Naruto stood at the same crossroads.
According to Minato, Naruto already satisfied the three core requirements:
Massive chakra reserves
Mastery of sealing techniques
Complete understanding of the Flying Thunder God formula
All that remained was to:
Mark a location with the formula seal
Sense the chakra embedded in the seal
Trigger resonance between his chakra and the seal
Instantly teleport using the reverse-summoning effect
Naruto was confident he was ready.
Originally, he had planned to enter the Mirror World and fight a data clone of Minato, hoping to experience combat against a Flying Thunder God user.
But the system had denied the request—
insufficient data for Minato's clone.
Next, Naruto attempted to mark formula seals on combat puppets inside the Mirror World.
Each time, however, the system forcibly ended the battle, spamming the same message:
"Please complete data collection first."
Left with no other option, Naruto returned to the seal space.
Here, ninja tools could be projected, and most techniques functioned normally.
That's when his gaze fell on the giant iron gate sealing Kurama.
He tested it—ninjutsu struck sparks, and even scraping it with a kunai left visible marks.
Yet no matter what he tried, the Flying Thunder God formula refused to adhere.
That was when Minato finally explained the truth.
The iron gate wasn't real.
It was merely a spiritual projection of the seal itself, not a physical object.
"Looks like I need a way to avoid surveillance," Naruto thought coldly.
If he wanted to seriously train Flying Thunder God, he needed a truly unobserved space.
After a moment, he turned to Minato.
"All right, Dad. Let's put Flying Thunder God aside for now," Naruto said.
"Let's work on adding nature transformation to the Rasengan."
Minato's expression immediately became awkward.
"Uh… Naruto… I actually tried that when I was alive. I even asked Jiraiya-sensei for help."
"But even Jiraiya couldn't manage it, despite mastering the Rasengan. Later, we tried incorporating Sage Mode to forcibly enhance chakra control…"
He scratched his head sheepishly.
"But by then, I was already Hokage. I didn't have time to train at Mount Myōboku."
"And… well. You know what happened after that."
Minato sighed.
"So when it comes to adding elemental change to the Rasengan… we're basically starting from the same point."
Naruto frowned.
In the original timeline, Naruto eventually added Wind Release to the Rasengan, creating the Rasenshuriken.
But the early version was deeply flawed:
It couldn't be thrown
It required close-range use
It damaged the user almost as much as the enemy
Only after mastering Sage Mode did Naruto perfect a throwable Rasenshuriken.
Another setback.
His training plan collapsed again.
Suddenly, Kurama, who had been quietly resting, propped himself up.
"Tch… Fourth Hokage, you're overthinking it," he growled.
"Why not just make the Rasengan bigger? That's how Tailed Beast Bombs work—the more chakra, the more power."
Minato froze.
"Something like a Tailed Beast Bomb requires absurd chakra levels, and my chakra—"
He stopped mid-sentence.
Naruto was different.
An Uzumaki.
Chakra wasn't a problem.
Naruto's eyes lit up.
Kurama's words made perfect sense.
In canon, the Rasengan already had variants like the Giant Rasengan.
Inspired, Naruto immediately formed a standard Rasengan in his palm and began pouring in more chakra.
Inside the seal space, the Rasengan expanded rapidly—growing from fist-sized to the size of a soccer ball, with no sign of stabilizing.
Then an even bolder idea crossed his mind.
Naruto began compressing the chakra—using the same principles Kurama had taught him for forming a Tailed Beast Bomb.
Minato instantly sensed something was wrong.
Just as he opened his mouth to stop him, Kurama raised a claw in warning.
Don't interrupt.
The massive Rasengan began to shrink—
—but its chakra density skyrocketed.
Within seconds, it compressed back to fist-size.
Except now, it was pitch black.
Naruto could feel it.
The chakra density inside the sphere had reached an incomprehensible level.
Chaotic, rotating rings orbited its surface—like planetary rings torn from a cosmic body.
It no longer resembled a Rasengan.
It looked like a calamity waiting to happen.
Minato's eyes went wide.
Kurama raised an eyebrow.
Naruto stared at the black sphere in his hand and muttered softly:
"What kind of Rasengan… is this?"
(End of Chapter)
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