Ultimately, the power ceiling of the Demon Slayer world was simply too low. The emergence of "demons" already represented the pinnacle of that world's strength. No matter how hard Ryūhei Ubuyashiki tried, he couldn't defy the limits of that world's power system.
Or rather, Ryūhei Ubuyashiki had already been pushing against those limits with great difficulty.
Breaking free from the constraints of a world's power system required immense effort and opportunity. But Ubuyashiki, having committed countless atrocities and earning retribution, was targeted by the Hegemon of Terror, leaving him no chance to continue his attempts at transcendence.
Tap, tap, tap.
Shinji's fingertips lightly drummed on the desk as his thoughts began to wander.
After organizing the materials on "Breathing Techniques" and "demons," Shinji realized that these two systems, representing good and evil in the Demon Slayer world, were fundamentally about developing the body.
The difference lay in their approach: "Breathing Techniques" respected the body's natural structure, unlocking the latent power within the human framework.
"Demons," however, completely abandoned the body's original structure, forsaking the balanced functions of organs. Rooted in flesh, they grew stronger by plundering the blood and flesh of other lives.
The former was conservative, emphasizing technique. The latter was radical, unrestrained by taboos.
In terms of development, the generations of Demon Slayer Corps swordsmen had clearly outdone demons—who constantly battled their bloodthirsty instincts—in refining "Breathing Techniques."
Yet, the overall strength of demons undeniably surpassed that of the swordsmen.
From Shinji's perspective, this was because "Breathing Techniques" could only harness the power of the body's organs, while "demons" could tap into the power at the cellular level.
The diverse "Blood Demon Arts" were, in essence, a qualitative transformation resulting from the resonance of cellular-level power combined with mental energy.
"'Demons' do have their merits. Their utilization of cellular-level power clearly surpasses that of 'Breathing Techniques,'" Shinji mused, his gaze growing profound.
In his view, the demons' approach to bodily development was overly radical and crude.
This turned demons into inhuman entities. While they gained immense power, they were also shackled by heavy limitations, set on an irreversible path of bloodthirst.
If he could eliminate the negative aspects of demons—such as their bloodthirsty instincts and inability to withstand sunlight—while retaining their techniques for harnessing cellular power, then the power of "demons" wouldn't be entirely useless.
Shinji even considered combining "Breathing Techniques" with "demons."
One system tapped into the power of organs, the other into cellular power. Merging the two would undoubtedly produce a synergistic effect greater than the sum of its parts.
This wasn't a whimsical idea. Figures like the demonized Tanjiro, an Upper Rank One, or Kokushibo, Upper Rank Two, had combined "Breathing Techniques" with the flesh-based power of "demons," effortlessly becoming Upper Rank demons.
Even someone like Kaigaku, a less remarkable swordsman in the Demon Slayer Corps, smoothly rose to the Upper Ranks after becoming a demon.
In Shinji's view, once someone became a demon, their human bodily structure was severely disrupted, weakening the efficacy of "Breathing Techniques."
Even so, combining the two systems significantly boosted strength, suggesting that a perfect fusion of the two could yield truly remarkable results.
Tap, tap, tap.
The rhythm of his tapping fingers quickened, mirroring Shinji's racing thoughts.
"There's great potential here!" he thought.
Before reaching the "Six Paths level," a shinobi's strength still hinged on their body and chakra.
Transplanting the First Hokage's cells involved acquiring the potent Yang Release chakra of Asura Ōtsutsuki contained within those cells to enhance one's physique—an elevation of chakra.
Meanwhile, merging "Breathing Techniques" and "demons" was about developing the body, improving the utilization of organs and cells.
The two approaches didn't conflict—rather, they complemented each other!
Since chakra was the product of combining physical and mental energy, strengthening the body could significantly enhance both the quantity and quality of chakra.
Shinji even believed that a sufficiently strong physique would provide substantial support for achieving the "Six Paths level." Forcibly becoming the Ten-Tails' Jinchuriki was an excruciating process—someone like Obito nearly passed out from the pain. Without a robust physique, even with the Ten-Tails before you, becoming its Jinchuriki was a highly risky endeavor.
As his thoughts churned, the vague ideas in Shinji's mind gradually took shape.
He pulled out a notebook and began outlining his plan.
The process was divided into three steps.
Step One: Optimize "Breathing Techniques."
With a shinobi's physical resilience, there was immense room for improving "Breathing Techniques." Moreover, the sheer variety of Breathing Techniques was excessive. Shinji could draw on the strengths of all schools and, through this optimization, create the ultimate "Breathing Technique."
Step Two: Transform "Demons."
The drawbacks of demons were too significant. Even if it meant sacrificing some of their vitality, Shinji had to completely eliminate those flaws.
At the very least, he needed to preserve the human body's structure and the balance of its organs. He couldn't abandon the essence for the sake of power, becoming a slave to flesh like demons.
Step Three: Combine "Breathing Techniques" with the Flesh Power of "Demons."
If the first two steps were successful, the third wouldn't be too difficult. After all, Shinji had trained in "Breathing Techniques" and studied medical ninjutsu, giving him an intimate understanding of the body's organs and physiological structures. Many obstacles that were insurmountable for swordsmen or demons were trivial to him.
For optimizing "Breathing Techniques," Shinji already had a direction, or more accurately, he was inspired by Ryūhei Ubuyashiki's notes.
Ubuyashiki had also considered optimizing "Breathing Techniques," believing that the multitude of techniques only scattered the swordsmen's focus. Only by having everyone specialize in a single "Breathing Technique" could it be refined to perfection.
Yet, in his lifetime, he never managed to create a perfect "Breathing Technique."
This wasn't solely due to Ubuyashiki's limited vision or skill—numerous objective constraints existed. For Shinji, however, those constraints were nonexistent.
With dojutsu, Shinji could effortlessly observe the airflow entering through the mouth and nose, its changes within the body, the flow of blood in the veins, and the force of muscle contractions.
With medical ninjutsu, he could use chakra to easily strengthen specific organs, airways, or blood vessels.
These were feats beyond the reach of swordsmen.