Seeking advantages and making the most of them had always been Shinji's way of doing things. So Ryuhei Ubuyashiki's failure didn't discourage him. If anything, the regrets between the lines of Ryuhei's notes stirred a fierce determination in Shinji to see this through.
After giving it a long thought, Shinji decided to set aside the first step—optimizing the Breathing Technique—and start with the second: modifying demons.
Ryuhei Ubuyashiki had made plenty of attempts to modify demons, given that he himself had been the Demon King. Shinji saw great promise in several of the directions Ryuhei had explored. If he built on that work, he might see results before long!
Once he'd made up his mind, Shinji got straight to work.
First, he went through all of Ryuhei Ubuyashiki's research notes related to demons.
In a sense, the creation of demons had been an accident—a product born from the chance convergence of a special potion and a special bloodline. A cruel joke played by fate!
Ryuhei Ubuyashiki had already tortured the formula for that special potion out of Muzan Kibutsuji.
Not only the formula, but also plenty of the various ingredients had been stockpiled in Ryuhei Ubuyashiki's laboratory. Shinji had hauled the lot back with him.
As for the special bloodline, it belonged to the Ubuyashiki Clan.
To eliminate any future threat, Ryuhei Ubuyashiki had personally wiped out his own clan once he discovered the truth. But in order to research demons, he had preserved blood and cell samples from the Ubuyashiki Clan during the massacre. Shinji had brought those back to the Shinobi World as well.
So if Shinji wanted to, he could follow the notes and cultivate an entirely new demon in his own laboratory.
Of course.
He hadn't gone to all this trouble to create a new demon.