While training Tanjiro, Charles was also studying Sun Breathing.
At Charles's level, he only needed to spar with Tanjiro to learn Sun Breathing's sword forms and its most fundamental breathing method.
As for the sword forms, he could pass. They were a set of techniques specialized against Muzan and not very practical for Charles.
Sun Breathing has thirteen forms precisely because the first twelve are meant to sever Muzan's five brains and seven hearts, and the thirteenth form finishes him completely.
The issue is that Yoriichi Tsugikuni had already completed the first twelve before he ever met Muzan, and as soon as he saw Muzan, he immediately completed the long-conceived thirteenth.
So when Yoriichi said he came to this world to slay Muzan, it wasn't without reason.
No wonder Muzan's psychological shadow is so large. Faced with a man tailor-made to counter him in every way—an outright natural enemy—who could endure it?
What mattered most was the Breathing itself. Charles carefully observed how Tanjiro's organs operated and grasped the core of the method.
Mobilizing large muscle groups and coordinating the operation of the organs together, Charles completed his first run of a Breathing Style.
While running the Breathing, Charles was surprised to discover he was drawing in not only oxygen but natural mana as well!
Natural mana is a power widely found across many worlds, distributed throughout the natural environment as energy particles or life force.
It may go by other names elsewhere, but its essence is the same.
For most humans, noticing it is hard enough, let alone using it.
A magus like Charles can only absorb natural mana via a mana organ such as a mana source, mix it with the soul and the body's strength, convert it into personal mana, and then use it.
That's because natural mana has strong assimilative and corrosive properties; a human body can't directly withstand it. Excessive erosion by natural mana can literally be fatal.
Breathing Styles allow a person to absorb minute amounts of natural mana, which is the fundamental reason they strengthen the body.
The so-called Marks should be the external manifestation of a swordsman absorbing large quantities of natural mana.
While the potent mana grants tremendous strength, excessive erosion also slashes their lifespan.
That is why, Yoriichi aside, all swordsmen who awakened Marks died before twenty-five.
As for Yoriichi, born with a Mark, he was likely one who could perfectly fuse with natural mana—nature itself gave him a cheat.
Naturally, Charles wouldn't copy Sun Breathing wholesale; he tailored it to himself.
Other swordsmen used Breathing Styles mimicking Sun Breathing because they couldn't withstand natural mana the way Yoriichi could.
But Charles was different; his constitution had long since diverged from a normal human's.
A magus's mana source can absorb natural mana by design, and Charles's world magic had wildly amplified his resistance to its erosion.
So rather than degrading Sun Breathing like other swordsmen, Charles was trying to increase the Breathing's intensity.
Even experimenting with a new Breathing, he was far beyond a just-beginning Tanjiro. Training mostly amounted to Charles pummeling him.
After night fell, Charles carried Tanjiro—who couldn't even twitch his pinky—back to their lodgings.
Hanako had already prepared hot bathwater as Charles instructed, and the stripped-bare Tanjiro was dropped straight into a tub steeped with medicinal herbs.
After the herbal bath and a hearty meal, Tanjiro came back to life.
The next day, they set out again. This time Charles didn't have Tanjiro pull the cart but had him carry the box with Nezuko inside.
The day before had snow and no sunlight; from here on, they had to account for Nezuko's inability to be exposed to the sun.
Charles built the box himself, with soft lining to keep Nezuko comfortable inside.
Their target for the day was the mountain opposite the Kamado home. Setting out early, Tanjiro began a new round of physical conditioning.
Running through the mountain forest with the box on his back, Tanjiro's nerves were taut. He couldn't afford not to be tense, because…
Whoosh! Whoosh! Whoosh!
Three sharp swishes split the air in rapid succession. Tanjiro stopped short and leaned back; three stones skimmed past him and punched deep holes into the snow.
He immediately rolled to dodge another stone.
Today's training, beyond physical conditioning, focused on reaction speed.
Battles between humans and demons are profoundly unequal. Beyond disparities in physical attributes, the key is that demons regenerate quickly, while human injuries immediately sap combat power.
Reaction, therefore, is critical; it's a decisive quality when fighting demons.
The stones Charles threw pressed right up against Tanjiro's reaction limit—and hurt like hell on impact.
If his footwork slowed even a bit, or his breathing slipped, he'd be hit. Relax, and he couldn't possibly dodge the next stone. By the time he reached the next village, he was black-and-blue all over.
Charles and Stella appeared at his side, Charles with a resigned Hanako Kamado tucked under his arm.
Compared with the panting Tanjiro, the two weren't flushed or winded in the slightest.
Tanjiro downed the vial Charles handed him, and the swelling on his face began to fade.
Setting Hanako down, Charles said to Tanjiro, "Go ask if there's a place we can stay. Something seems to have happened here, so I'm going to take a look."
As soon as he entered the village, Charles could feel the villagers' fear.
Given this world, it was likely something to do with demons.
Charles had thought the first demon on the road would be the Temple Demon. He hadn't expected to encounter a man-eater before Mount Sagiri.
Yes, Mount Sagiri was indeed on Charles's itinerary. It was the residence of the former Water Hashira, now trainer, Sakonji Urokodaki.
Although Tanjiro knew Sun Breathing, his command of Breathing was far too shallow. There was still much Charles wanted to consult that seasoned swordsman about.
Charles also wanted to use him to establish contact with the Demon Slayer Corps. With their intelligence network across the archipelago, combined with the crucial intel Charles already had, he could make the journey far more efficient and reduce the Corps' losses.
Those in the Demon Slayer Corps—Charles hoped they could live happily in a world without demons.
That should've been achieved through Giyu Tomioka, but his inability to communicate had been worse than expected.
Before heading to Mount Sagiri, though, it was time to see what exactly had happened here.
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