Perfect Fusion!
The limit of being in a world barren of Sword Breath didn't only restrict Leo's advancement, but it also turned his True Sword Breath into a consumable resource, 'True Breathing' was impossible without a source of replenishment.
If the current Leo ended up encountering any situation that actually required the use of Sword Breath, he barely had less than 7 hours' worth of reserve. Most importantly, running out didn't end up with being powerless, it was practically a death sentence for a mutant like him…
"Let's keep going!"
For the sake of self-sufficiency, Leo could only proceed. He wasn't just betting on his life, rather, he had enough confidence in his success.
To an observer who didn't experience what he was experiencing that might not seem to be the case. Leo, however, could tell how smooth this consumption was. In fact, he could no longer even call it consumption… This was a fusion!