The three great dōjutsu of the ninja world: the Byakugan, the Sharingan, and the Rinnegan.
In the entire ninja world, there was probably no one who understood the abilities of these three dōjutsu better than Naruto Uzumaki. He knew the source of their power and was even aware of the Tenseigan, the Byakugan's evolved form.
Hiashi Hyuga, however, did not know this. He led Naruto through a series of twists and turns into a secret room. From an iron box inside, he retrieved a scroll.
"This is the Byakugan that you helped us retrieve back then, Lord Hokage."
Hiashi held the scroll tightly, his palm trembling slightly, a clear sign of his inner turmoil. His next words were tied to the difficult decision he was about to make. After a long moment of contemplation, he handed the scroll to Naruto.
"If you can give Hinata happiness, Lord Hokage," he said in a deep voice, "then this Byakugan should be returned to your hands."
It was a decision made on the spot. If Hinata were to marry Naruto after she turned eighteen, the clan would surely be upset about the Hyuga's kekkei genkai leaving the family. But if Naruto was willing to accept the transplant of this Byakugan, it might greatly ease the conflict their marriage would cause.
Besides, Naruto was the one who retrieved this Byakugan in the first place. There was no issue with him choosing to transplant it himself. And there was already a precedent for such a thing. That man, Kakashi Hatake, had transplanted a Sharingan on the battlefield. The Uchiha clan only dropped the matter because he was the Fourth Hokage's disciple.
Naruto looked deeply at Hiashi, then reached out and took the scroll. He slowly removed the Byakugan… It was Ao's eye.
He hadn't realized that, during the period when his memory was sealed, he had already killed the man who would become the captain of the Sensor Division in the future Fourth Great Ninja War.
The tearing pain in his brain struck again.
They had been talking for so long that the Yang Power the Nine-Tails could provide for healing had already been used up. If he didn't do something now, his brain would have to seal the memories away again.
Naruto suddenly dropped to one knee.
Hiashi rushed toward him in a panic, his expression nervous as he tried to help him up. "Lord Hokage, your body..."
In his Byakugan's vision, the dense spiritual energy in Naruto's brain was practically leaking out. It was a terrifying sight. Even having experienced the Great Ninja War and witnessed many powerful ninja, Hiashi had never seen anything that could compare to the power contained within Naruto.
"Naruto, try that other jutsu of Orochimaru's!" the Nine-Tails said from within the seal. It looked worried but was still thinking rationally. "The Sealing Jutsu: Devour that Orochimaru developed is probably not that simple. If it were just for absorbing chakra, he could have just told you to seize the Rinnegan with your own power!"
The Rinnegan itself possessed the ability to absorb jutsu and chakra. If Orochimaru's technique had the same function, there would have been no need for Naruto to learn it.
"Understood. I suspected that long ago," Naruto thought back, forcing a smile. He reached out, wrapped his hand around the Byakugan, and suddenly clenched his fist. "Sealing Jutsu: Devour!"
As the reincarnation of Asura's chakra, Naruto knew exactly what it was capable of. Besides awakening Yang Power, its most commendable feature was its vast quantity of chakra. He was also well aware that he shouldn't have the Wood Style kekkei genkai. Even in the future, when he transplanted the arm made from the First Hokage's cells, he never awakened Wood Style.
On that point, he was worse off than both Obito Uchiha and Danzo Shimura, who both awakened Wood Style after transplanting the First Hokage's cells. Naruto suspected it was because the amount of the First Hokage's cells in his body was too small and had been assimilated by his own Uzumaki bloodline. It enhanced his recovery ability but offered no possibility of awakening Wood Style.
And yet, now he had awakened it. The only explanation could be the strange nature of the Sealing Jutsu: Devour. Knowing Orochimaru, the jutsu he developed could very well devour a kekkei genkai. After all, that man had a strong liking for bodies with powerful bloodline traits.
The Byakugan in his palm turned into a puff of smoke and dissipated.
Naruto calmly felt his eyes begin to change. He could clearly sense the spiritual energy in his brain converging toward them. In the seal space, the Nine-Tails looked at Naruto's eyes in surprise. They had unknowingly deepened into a radiant, celestial blue — far brighter than before.
"What's going on? Why does it look a bit like that guy on the moon…" Kurama muttered. It looked like the Tenseigan. But he had only devoured an ordinary Byakugan.
"Perhaps this isn't so surprising," Naruto replied, the pain in his head finally receding. His face gradually calmed. "The Byakugan I just devoured guided me to unlock the kekkei genkai, but it also consumed all of my excess spiritual energy."
A peak-level Naruto, combined with the gifted energy of nearly the entire Nine-Tails—the spiritual energy they possessed together was immense⁷, not inferior to the combined total of the Otsutsuki descendants on the moon. The Byakugan was a dōjutsu unlocked through bloodline inheritance and the gathering of spiritual energy. To make it stronger required a continuous supply of that energy.
Now, all the spiritual energy in Naruto's brain had vanished without a trace, turned into nourishment for cultivating the Byakugan. The very spiritual energy that his future self brought back, which would have been enough for him to rival the Sage of Six Paths at his peak, was gone. That was the power that could have given him a fighting chance against Kaguya.
In the end, all it got him was a pair of Tenseigan. This dōjutsu was indeed powerful, but it was far from being as precious as the spiritual energy that had been tearing his brain apart.
"Naruto..." The Nine-Tails looked at Naruto's radiant blue eyes, its thoughts drifting. "Do you remember the old toad's prophecy? The blue-eyed boy who laughs and calls the names of the nine beasts…"
"..."
Naruto's heart tightened. Many people called him the Child of the Prophecy because he was the only ninja to befriend all nine Tailed Beasts. Now, the final part of that prophecy had come true. His eyes had changed from azure blue to celestial blue. He had truly become the "blue-eyed boy" from the Great Toad Sage's vision, the one who could call the names of the nine beasts.
The time to truly change the ninja world had just begun. The path he was walking now was the right one.
Back in the real world, Naruto slowly opened his eyes. They were divine blue and seemed to contain a beautiful galaxy, making it hard to look away.
Bang!
The floor beneath his feet cracked and turned to powder.
Hiashi jumped back in surprise, realizing that a single attack from Naruto could have easily and severely injured him.
"Lord Hokage!"
"Sorry," Naruto smiled apologetically at Hiashi. "I can't control my power very well yet."
He slowly closed his eyes, then opened them again. The otherworldly blue faded, returning to their familiar azure blue.
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Bonus:
Naruto: Why did you turn my eyes blue?
Nong Mo (Author): I'm patching a plot hole!
Naruto: Is this your plot hole?
Nong Mo: ...Is it?
Naruto: Hell no! You're just a fanfiction writer, how can you be so shameless? You know nothing about Naruto!
Nong Mo: ...
Naruto: The "blue-eyed boy" in the Great Toad's prophecy is Kishimoto's plot hole, not yours!
Nong Mo: In that case, should I recommend a book?
...
P.S. I think the ff author thought blue eyes were too common in Naruto and considered it a plot hole.