After synthesizing his current physical examination data with Karin's previous metrics, Shinji suppressed the urge to immediately transplant the First Hokage's cells.
Pursuing the Rinnegan was not something to rush—haste would only lead to failure!
As long as he prepared every step meticulously, achieving his goal steadily was assured, and there was no need to take reckless risks.
Setting aside thoughts of the surgery, he shifted his focus to organizing the "legacy" from the Demon Slayer world.
Through meticulous classification and analysis, Shinji confirmed he had collected thirty types of Breathing Techniques. Among them was the "Sun Breathing," the origin of all Breathing Techniques, alongside various derivative techniques created by later generations.
In addition to the Breathing Techniques, there were 742 volumes of related training methods, insights, and experiences.
Of these, nine volumes delved into advanced states of Breathing Techniques, such as the "Mark" and the "Transparent World." Two of these were authored by Ryūhei Ubuyashiki himself, comprehensively summarizing predecessors' understanding, research, and visions for Breathing Techniques, complete with his own annotations. These were exceptionally valuable references.
Forget mastering all this material—even a cursory review greatly benefited Shinji. Combined with his knowledge of medical ninjutsu, he gained a clearer direction for optimizing Breathing Techniques.
"The swordsmen's development of Breathing Techniques never reached its peak," he mused, pausing before continuing silently, "It's not their fault. Their bodies simply couldn't support further exploration of Breathing Techniques. After all, just activating the 'Mark' was enough to cause their geniuses to die young. But I'm different."
Shinji was well aware of his advantages.
Chakra granted shinobi far greater vitality than swordsmen, and the existence of medical ninjutsu provided opportunities to push the limits of the human body.
In his view, optimizing Breathing Techniques could significantly enhance his combat strength, at least until reaching the "Six Paths level," allowing him to maximize his body's potential.
Beyond the "Six Paths level," the body begins to undergo chakra transformation—or rather, energy digitization. At that point, techniques like the "Eight Gates Released Formation" or "Breathing Techniques," which rely on the physical body's flesh and organs, lose much of their effectiveness.
Take Kaguya Ōtsutsuki, for example. She had no need to train in such physical techniques. Her "Eighty Gods Vacuum Attack" surpassed even Might Guy's ultimate technique, "Night Guy," unleashed through the "Gate of Death."
This wasn't because Kaguya's physical techniques outclassed Guy's, but because, as a wielder of the "Bloodline Encompassing," her body had already undergone chakra transformation, transcending the limitations of flesh. Her lack of finesse in physical combat didn't hinder her ability to fully unleash her power at will.
After organizing the materials related to Breathing Techniques, Shinji turned his attention to data concerning "demons."
This section was, in truth, more intriguing to him than Breathing Techniques, as immortality was an irresistible temptation for any living being.
However, after thoroughly reviewing the demon-related materials, Shinji felt a faint sense of disappointment.
The potential of demons wasn't as great as he had imagined. Their strength was, to some extent, entirely dependent on the Demon King.
In other words, becoming a demon essentially meant becoming a servant of the Demon King.
Back then, Ryūhei Ubuyashiki had first gravely injured and captured Muzan Kibutsuji, using some method to extract and absorb the Demon King's power to become a demon himself. Only this allowed him to escape the Demon King's control.
Had he become a demon through any other means, even as the "Lampbearer," he would still have been bound to the Demon King.
Ordinary demons, while granted long lifespans, had their strength capped by the Demon King, who held near-absolute control over their life and death.
The Demon King's strength, by contrast, faced fewer limitations.
Yet, according to Ryūhei Ubuyashiki's notes, even the Demon King's power would plateau after reaching a certain threshold. No matter how many lives they consumed or how much blood they devoured, their strength growth would slow to a crawl.
Ryūhei Ubuyashiki himself was a prime example.
He had eradicated countless civilizations and slaughtered an uncountable number of beings, yet his power never broke through the limits of what a "demon" could achieve. No matter how brutally he plundered, he remained confined by the power of flesh, his efforts merely inching toward a marginally better mastery of it.
Thus, Ryūhei Ubuyashiki served as the most direct proof.
Even abandoning human dignity, bodily structure, and form, the ultimate limit of the Demon King's power was merely Ryūhei Ubuyashiki's level!
By comparison, chakra transformation at the "Six Paths level," digitizing one's soul and body through chakra to pursue a higher form of existence, was clearly a tier above the Demon King's power.
In the shinobi world, with chakra at his disposal, forsaking the potential to achieve the "Six Paths level" or even the greater "Bloodline Encompassing" to instead destroy one's body and become a "demon" was plainly foolish.
Moreover, according to Ryūhei Ubuyashiki's research, demons couldn't achieve true immortality.
Low-tier demons, over their long lives, gradually lost their sense of self, shedding their human will until they became bloodthirsty beasts.
Higher-tier demons experienced this loss more slowly, but it was still inevitable.
Even the Demon King himself wasn't immune.
In one of his notebooks, Ryūhei Ubuyashiki noted that without the "Lamp" constantly reminding him, he wouldn't even have noticed that his human will was being eroded bit by bit.
He theorized that this was due to the body's transformation, which altered his senses, thought patterns, pain perception, and physiological responses, making him fundamentally different. This transformation wasn't just physical but psychological as well.
The bloodthirsty instinct would erode his original morals and values, gradually reshaping him until he became an entirely different being from his former self.
"Hmph," Shinji chuckled, reading Ryūhei Ubuyashiki's analysis.
As a medical shinobi, Shinji could see that Ubuyashiki's analysis wasn't comprehensive. The key reason demons slowly lost their human will was that the power of their flesh harbored its own consciousness, with bloodthirst being its outward manifestation.
The stronger the flesh's power, the more dominant this ingrained will became.
As the human will waned in this tug-of-war, it was gradually suppressed and eventually eradicated, reducing the individual to a monster driven solely by the instincts of the flesh.