"Did you know about these things very early on, Fourth Mizukage?"
"Very early? No. At the time, I only had some guesses I made on my own based on known information."
Rei Kaguya shook his head, pushed the Tailed Beast chakra circling around him toward Naruto Uzumaki, and answered calmly.
"If natural energy is something that exists commonly on every planet the Otsutsuki have descended upon, then given their degree of control over chakra, as well as their endless pursuit of power and higher levels of life, there is no reason they would not have incorporated it into their own chakra system."
"But judging from Kaguya Otsutsuki's memories of the Otsutsuki main family's members, the members of the Otsutsuki clan never mastered or understood this power—a power that can combine with chakra and grant mortals the possibility of defying their betters and damaging their noble bodies."
"From a probability standpoint, the chance that they simply never discovered natural energy's existence is infinitely close to zero."
"And since the members of the Otsutsuki main family did not know what natural energy was, the problem must have lain with Kaguya Otsutsuki, who failed to completely harvest all of the shinobi world's chakra, was even counter-killed, and had the Ten-Tails stripped from her body—or rather, with the act of returning the chakra gathered by the Ten-Tails back to the earth."
Naruto Uzumaki looked at the Tailed Beast chakra that had begun revolving around him on its own and asked with clear eyes, "Then what should I do now?"
Rei pointed at his own head, then at his heart. His tone was as calm as an ancient well.
"Chakra is the product of spiritual and life energy combined. That means each person's chakra contains their own will, memories, and information… The Tailed Beasts are aggregations of chakra. Naturally, their chakra contains even more information about this planet."
"With the Ten-Tails' power, you can do what true Otsutsuki do and draw this information, along with information that was once engraved inside the Ten-Tails, into your own mind. I need you to use the information that was intentionally or unintentionally recorded inside chakra to find the source of natural energy's birth, as well as information on Isshiki Otsutsuki, who was ambushed by Kaguya and offered to the Ten-Tails back then… Can you do it?"
"Leave it to me."
Naruto Uzumaki gave Rei a thumbs-up, making a reassuring gesture. Then he took a deep breath and absorbed all the Tailed Beast chakra revolving around him into his body.
In an instant, Naruto Uzumaki's consciousness body changed appearance once again.
From the form that resembled his state during the first battle at the Valley of the End, when he was wrapped in a Tailed Beast cloak, he transformed at once into the appearance he had taken after receiving the Sage of Six Paths' legacy and obtaining Six Paths power.
A golden chakra cloak covered his body. The nine Truth-Seeking Balls that symbolized the pinnacle of Yin-Yang Release slowly formed behind his head. Then, the nine Truth-Seeking Balls began changing shape, gradually revealing the forms of each of the Nine Tailed Beasts.
At the same time, traits that only appeared among the Otsutsuki clan began emerging upon Naruto Uzumaki's consciousness body.
His blond hair slowly turned pure white. Horns like the Sage of Six Paths' grew from his forehead. And his blue eyes, under the effect of that immense chakra, gradually turned into a pale blue distinct from both Rei Kaguya's Tenseigan and an ordinary Byakugan.
At the same time as his consciousness body changed form, Naruto Uzumaki plunged headfirst into the vast, surging sea of memories belonging to the Tailed Beasts.
There, he found the first fragment of information worthy of his attention.
"Kaguya!? Why? Why are you doing this—"
"Have you betrayed me!? Are you truly going to betray me, betray the entire Otsutsuki clan—Uwaaaaaaaaaah!!!"
Kaguya Otsutsuki ambushed Isshiki Otsutsuki, who had been waiting for her to sacrifice herself to the Ten-Tails. She reversed their positions and threw the heavily injured Isshiki toward the Divine Tree Ten-Tails, which had already sunk its roots deep into the earth.
Amid his miserable screams, most of Isshiki's body was devoured by the Ten-Tails. As for the remaining ruined portion, it instantly shrank under his unique dojutsu, Sukunahikona, and fled into the distance in a wretched state.
The Ten-Tails, having received an incomplete sacrifice, let out an earth-shaking roar.
Although most of Isshiki Otsutsuki's body had been enough to mature it into its complete form, the information contained in Isshiki's chakra—both in quality and quantity far surpassing that of all other living beings—was firmly recorded within the Ten-Tails' body.
After the chakra fruit matured, Kaguya Otsutsuki swallowed it and obtained that information. But because she firmly believed Isshiki was already dead, she did not take it too seriously. On top of that, Isshiki hid his chakra and existence in an extremely concealed manner. Only then was he able to escape Kaguya Otsutsuki's gaze and survive to the present day.
But to Naruto Uzumaki, who knew very well that Isshiki had not died, this information, which recorded almost everything about Isshiki Otsutsuki, was already enough to give him a chance to catch Isshiki's true body.
After firmly recording the other party's chakra information, Naruto Uzumaki continued diving deeper into the sea of memories.
Then, at a somewhat unexpected point in time, he discovered the original birth of natural energy.
After Kaguya Otsutsuki devoured the chakra fruit and obtained great power, she pacified the shinobi world's earliest age of chaos like a god, became the Rabbit Goddess worshiped and revered by the people, and, amid everyone's blessings, gave birth to the two brothers, Hamura Otsutsuki and Hagoromo Otsutsuki.
To the people who worshiped the Rabbit Goddess, this was not something strange.
After all, in their eyes, Kaguya Otsutsuki was the Rabbit Goddess who could create and destroy the world. Even if she conceived offspring alone, that was merely another manifestation of a god's supreme authority.
But to Kaguya Otsutsuki, who had swallowed the chakra fruit produced by the mature Divine Tree Ten-Tails rooted in the shinobi world, this was an act of immense symbolic significance, and one that violated the traditions of the Otsutsuki clan.
She had given chakra that should have been monopolized and enjoyed by her alone to other people.
Even if those "other people" were children she had conceived by herself, children who shared her direct bloodline, there was no doubt that this was an act of distributing chakra once more to other lives born upon the planet known as the shinobi world.
And so, a kind of power that had never appeared during the Otsutsuki clan's conquest of the universe began to sprout within this planet, which had already been harvested once.
After Kaguya Otsutsuki was overthrown and sealed by her two children, Hagoromo Otsutsuki, the Sage of Six Paths who inherited the Ten-Tails, made a decision unprecedented before him and perhaps never to be repeated after him.
He divided the Ten-Tails' chakra, vast enough to create and destroy the world, into nine portions. He created life-forms made entirely of chakra, life-forms that possessed consciousness of their own—the Nine Tailed Beasts—and taught the humans of the shinobi world how to use chakra.
And so, chakra that had once been plundered, once been harvested, once been lost, returned to this planet that should have moved toward deathly silence.
And so, the seed that had once been only an accident, along with the full return of the chakra harvested by the Ten-Tails and the continued reproduction and development of humanity after mastering chakra, gained the opportunity to break through the soil and grow into a towering tree.
But even then, natural energy remained a power difficult for humans to control.
It had been born during the process in which harvested chakra was returned to the earth, so by nature, it possessed a trait of closeness with chakra.
But because it was born in the world after chakra had been harvested, natural energy also naturally carried certain traits that could counter chakra.
It was close to chakra, yet could counter chakra.
And so, naturally, natural energy became a power only an extremely small number of living beings could master.
Those living beings then gave it a brand-new name.
Sage Art.