All three present knew exactly what it meant if that sapling on Nawaki's shoulder grew into a full tree—it would mean the complete loss of his life force. He would forever become nothing more than an unconscious tree.
That was the true source of Tsunade's pain.. As the greatest medical-nin alive, she was utterly powerless before her grandfather's cells.
Hoshiyomi first comforted Tsunade, then carefully examined Nawaki, sinking into deep thought.
To be honest, Hoshiyomi's own understanding of Hashirama's cells was also vague. Back in his previous life, reading the manga, all he knew was that the cells were insanely powerful and prone to going berserk.
Still, compared to Tsunade and Hiruzen, Hoshiyomi had one advantage: he knew that transplanting Hashirama's cells did not always mean certain death. For that reason, he was a little more optimistic.
Forcing himself to calm down, Hoshiyomi began analyzing the situation bit by bit.
The first point to note was the three test subjects who had already died. According to Tsunade, all of them had died because the excessive use of chakra — or too much physical damage — had caused the Hashirama cells to go out of control. That was the first thing he had to be cautious about.
With that in mind, Hoshiyomi quickly told Tsunade:
"While you're using medical ninjutsu to sustain his life, make sure he doesn't wake up, or consume too much chakra on his own."
After reminding her of that, Hoshiyomi returned to thinking.
Later on, in the Uchiha clan, three survivors had transplanted Hashirama's cells. Yamato had Hashirama's cells in his body as well. Even Naruto in Boruto used a prosthetic arm cultivated from Hashirama Cells.
And of course, there was Danzo from the original story. He was perhaps the most pathetic example — depending entirely on the Hashirama cells in Shin's arm. After burning through the ten Sharingan with Izanagi, the Hashirama cells went berserk, forcing him to sever his own arm just to survive.
Wait a second… ten Sharingan…
As he thought of Danzo, a flash of inspiration suddenly crossed his mind. His thoughts turned to Madara and Obito.
Both of them had Sharingan and had transplanted Hashirama's cells. Yet, from beginning to end, neither of them had ever shown any backlash.
Even setting Madara aside as Indra's reincarnation, Obito was an obvious example. Originally, he had been just an ordinary dead-last, yet after transplanting Hashirama's cells, he never once suffered from backlash.
Could it be… the ocular power of the Sharingan—or more specifically, Yin Release chakra—helped foster the fusion of Hashirama's cells?
It seemed like a feasible method, and there was a theoretical basis.
After all, the Uchiha and Senju were both descendants of Indra and Asura, the sons of the Sage of Six Paths — theoretically, they shared the same origin.
Furthermore, in the Naruto setting, Yin and Yang combined represent all creation. The reason test subjects underwent "tree transformation" was mainly because their bodies couldn't withstand the immense Yang Release and Wood Release energies contained within Hashirama's cells.
Just how strong was Hashirama's Yang Release? In the original story, Madara once remarked to Tsunade that her Byakugō Seal only put her at about the same level as Hashirama's normal state. That alone showed how monstrous his Yang Release truly was.
And Yang Release was also closely tied to Wood Release. In the manga, when Naruto entered his "golden form" (Nine-Tails Chakra Mode), Yamato even noted that the form's overwhelming vitality caused his own Wood Release to sprout and grow.
If this reasoning held true, then in order to suppress the Wood Release energy of Hashirama's cells, the first step would be to suppress their Yang Release.
And to counter excessive Yang Release, Yin Release was clearly the best option.
With that conclusion, Hoshiyomi instinctively turned his head toward Hiruzen and asked:
"Old man, do you know how to use Yin Release?"
Not waiting for Hiruzen to reply, Xingye suddenly smacked his own forehead. Was I an idiot? Hiruzen possesses all seven chakra natures, but don't I have them too?
With his Nature Transformation, he himself could also flexibly utilize all seven chakra natures.
Come to think of it, he had only himself to blame.
It wasn't as if he hadn't experimented in private before, trying to see what kind of effects Yin or Yang chakra might have on his sword energy. But after transforming them, the result was… nothing. No effect at all. Aside from the change in color, the sword energy he unleashed had no special properties.
Because it had no unique effects, Hoshiyomi had never used Yin/Yang sword aura in actual combat. Over time, he had practically forgotten that he even possessed those chakra nature.
But now, Nawaki's condition reminded him — he really did have them.
With that thought, Hoshiyomi quickly used Nature Transformation, converting his chakra into pure Yin.
He drew the Mikazuki Munechika, and as he poured chakra into it, the sword's surface shimmered with a sinister purple-black glow.
At that moment, not only Tsunade, but even Hiruzen Sarutobi was stunned.
Pure Yin chakra coating a weapon? How could that be possible?
In the shinobi world, Yin Release was always considered a mysterious chakra nature. The common understanding was that it could only be used to enhance one's mental energy, which could then be used to manipulate or assault the minds of others.
Even the Nara clan's shadow techniques — visible to the naked eye — were in essence the result of Yin chakra strengthening the mind, then using that mental energy to conntrol shadows.
If someone had told Hiruzen that pure Yin chakra could be imbued onto a weapon, the so-called "Professor of Ninjutsu" would have slapped that person across the face and told them to go back to the Academy to relearn the basics.
Yet here it was happening right before his very eyes. Even Hiruzen was left utterly stunned.
Hoshiyomi, however, had no time to care about Hiruzen's shock. He turned to Tsunade and said firmly:
"Trust me. Whatever I do next is for Nawaki's sake."
Before Tsunade could react, Hoshiyomi plunged Mikazuki Munechika straight into Nawaki's palm.
Tsunade gasped, covering her mouth in alarm, clearly frightened by his sudden action. She instinctively wanted to stop him, but when she saw the serious look in Hoshiyomi's eyes — no hint of jest — she hesitated, and finally held herself back.
She had no options left. All she could do now was entrust Nawaki's fate to Hoshiyomi's bizarre method, treating it as a desperate gamble.
Under Tsunade's nervous gaze, Hoshiyomi first froze for a moment, then a smile gradually spread across his face.
Because he could feel it—Hashirama's cells within Nawaki were reacting. They were ravenously devouring his Yin chakra.
Hoshiyomi had always been confident in the depth of his chakra reserves. He increased his output, letting the cells gorge themselves freely.
They consumed more than half his total chakra before finally quieting down.
Just as he had theorized, the Hashirama cells calmed after absorbing the Yin chakra—and even began to cooperate with Nawaki's natural cells, merging together.
Nawaki's life had been saved.