Uchiha Shinji faced a multitude of problems requiring resolution.
For instance, 'Upper Moon One,' 'Upper Moon Two,' and other high-ranking demons who survived the Land of Grass war were still lurking in the shinobi world, posing a significant threat.
Orochimaru was beginning to suspect Shinji's true motives for curing Kimimaro.
The Otsutsuki branch family on the moon, having lost another member, might take more extreme actions, such as prematurely triggering a world-ending catastrophe.
And so on.
But compared to these, the truly critical issue was the shinobi world's contamination!
As the mysterious figure had shouted "mutual destruction" while fleeing, Shinji had confirmed with his Susanoo Tomoshibi that the Demon Slayer world's contamination of the shinobi world had exceeded critical levels. Unless Shinji razed several nations centered around the Land of Grass, a collision between the Demon Slayer and shinobi worlds was inevitable.
This was one reason Shinji hadn't rushed to eliminate the remaining Upper Moon demons and allowed various shinobi world factions to research demons.
Compared to the contamination already inflicted, the demons' presence was hardly worth worrying about.
As the Lampbearer guarding the shinobi world, Shinji hadn't given up.
In his view, the mysterious figure's repeated invasions, bringing contamination to the shinobi world, couldn't have been aimed at mutual destruction from the start.
Such actions would be meaningless for both the mysterious figure and the Demon Slayer world!
There had to be a reason behind this behavior, one Shinji hadn't yet uncovered—a reason that might hold the key to saving the shinobi world.
Otherwise, within a year—or even sooner—the shinobi world could collide with the Demon Slayer world, annihilating both in the cosmos.
To uncover why the mysterious figure kept invading, Shinji needed to counter-invade the Demon Slayer world and root out the culprit.
But finding the mysterious figure, the 'Demon King,' in their own territory was no small feat!
Moreover, given the terrifying strength the mysterious figure displayed in the Land of Grass war, even if Shinji located them, without Nagato's involvement, he and his forces might not be able to handle the Demon King.
Thus, Shinji pursued a two-pronged approach: joining Tsunade's research team, hoping she'd swiftly develop a poison to counter demons' rapid regeneration, while accelerating his 'Rinnegan Plan' and 'Tenseigan Plan' to amass greater strength for the counter-invasion.
At this point, Shinji was racing against time.
If he failed to prepare for the counter-invasion or capture the mysterious figure before the worlds collided, the shinobi world would vanish!
Noon.
Instead of resting at the Shinobi Academy like other students, Shinji hurried to the secret laboratory beneath the hospital to assist with Tsunade's demon poison research.
During a routine meeting, Shizune reported, "Corrosive toxins have very limited effects on it."
Yakumo added, "Acidic toxins are also ineffective. Their cellular tissue has extraordinary regeneration. Even the most potent acidic toxins can't fundamentally harm them."
A senior medical ninja chimed in, "Intense light and incineration have some effect, but replicating sunlight's impact through poison isn't realistic!"
After a round of discussion, everyone's brows were furrowed.
The demons' regeneration ability was maddeningly perplexing.
At that moment, Shinji raised his hand.
Typically, he participated quietly, rarely offering opinions, but the discussion had sparked a memory of Tamayo's poisons developed against Muzan in the original Demon Slayer timeline.
One was an aging agent!
Cell division has limits, even for a Demon King-powered demon, who isn't truly immortal. Why not, like Tamayo, develop an aging agent to rapidly accelerate the mysterious figure's cellular aging, forcing constant cell division and plunging them into a weakened, aged state in a short time?
If that could be achieved, Shinji was confident he could defeat the mysterious figure.
Unlike the Demon Slayer Corps with their limited techniques, Shinji wielded ninjutsu, taijutsu, genjutsu, dojutsu, and sealing jutsu. He didn't need a poison to kill the mysterious figure outright—just one to weaken them.
Tsunade looked at Shinji. "Shinji, what's your idea?"
Without wasting words, Shinji shared his concept.
"Induce cellular aging and stimulate rapid cell division?"
After hearing Shinji's idea, Tsunade fell into thought.
They had been trapped in a rigid mindset, focusing solely on destroying demon cells. Shinji's suggestion opened a new avenue.
Soon, Tsunade broke the silence, standing and ordering, "Shizune, you're in charge of testing the 'monster's' cell division limits!"
The 'monster' was the codename for the demon chained in the lab.
Shizune stood immediately. "Yes!"
Tsunade turned to the senior medical ninja, Yakumo, and Shinji, instructing, "The medical team will collect toxins that accelerate cellular aging. Yakumo, Shinji, assist them. If the project succeeds, it'll be counted as completing an S-rank mission!"
The senior medical ninja, Yakumo, and Shinji stood together. "Yes!"
After leaving the lab, Yakumo said to Shinji, "Shinji, this is another rare S-rank mission. We've got to give it our all!"
"Mm-hmm!"
Shinji nodded enthusiastically on the surface but inwardly grumbled.
Clearly, Tsunade was deliberately grooming Yakumo. Normally, ninjas involved in such a project would earn an A-rank mission at best, especially non-core assistants like Yakumo and Shinji.
Tsunade's generous S-rank mission credit for Yakumo was meant to boost her credentials and contributions, paving the way for her future.
Having successfully integrated First Hokage cells, Yakumo was no longer an outsider in Tsunade's eyes, and her intent to nurture Yakumo was undisguised.
Barring any surprises, Yakumo had a 70-80% chance of becoming the next or the one after next Hokage.
As for Shinji, his multiple opportunities to participate in S-rank missions as an academy student were purely thanks to Yakumo's coattails.
Since they were both on the mission, it wouldn't do for Yakumo to receive S-rank credit while Shinji got only A-rank. Even if he agreed, the advisory elders backing him wouldn't.