Crossover: The Problematic Deity of Arcadia

— — — — — — 

The Sun Breathing.

It's the strongest of all breathing styles—the original one. It's the foundation; the Demon Slayer Corps was built on.

Five hundred years ago, during the Sengoku period, the Demon Slayer Corps was on the verge of collapse. Back then, even killing the weakest demon cost lives. 

But then everything changed when Yoriichi Tsugikuni introduced Sun Breathing to the Corps.

Suddenly, powerful swordsmen started popping up everywhere, and the death toll among demons began to skyrocket.

While Yoriichi was alive, demons didn't dare roam at night. Even if they had to, they crept around like scared rats, terrified the Corps might find them.

Why?

Because even Muzan Kibutsuji himself feared Yoriichi. The so-called Demon King had gone into hiding.

That era marked the peak of the Demon Slayer Corps—and it lasted a full eighty years.

Only after Yoriichi's death did Muzan resurface. And when he did, he went on a rampage, hunting down and slaughtering every swordsman who had learned Sun Breathing. He practically erased the technique from existence. That's when the decline began.

From then on, the Corps had to scrape by using derivative styles—adaptations of the lost Sun Breathing.

Still, those derivative breathing techniques changed the game. The old "throw lives at it until it dies" approach started shifting. Fighters could now take down demons in one-on-one duels—or even one-versus-many.

Eventually, a formal rule emerged: slay one Kizuki demon, and you earn the rank of Hashira.

So really, Sun Breathing was the lifeblood of the Corps—its power and prestige rose with it, and crumbled without it.

Which is why the return of Sun Breathing—thanks to Ryo—sent waves of excitement through the entire organization.

With an all-out war looming on the horizon, the reappearance of Sun Breathing wasn't just a power-up.

It was hope.

Even the ever-calm Stone Hashira, Gyomei Himejima, couldn't stay silent.

"Amitabha... Kyojuro, are you certain? Are you saying this is truly Sun Breathing?"

"Yes." Kyojuro Rengoku nodded solemnly."It matches perfectly with the ancient records of the Rengoku family. The breathing rhythm, the sword technique—it's like watching the gods dance. No doubt about it. This is Sun Breathing."

Hearing this, Gyomei pressed his palms together in reverence. "Amitabha."

He trusted Kyojuro—completely.

After all, the Rengoku family had served as Flame Hashira for generations. If anyone's records could be trusted, it was theirs.

Gyomei let out a long breath and even smiled slightly as he said: "Lord Yagami... I owe you both an apology. I was too hasty in my judgment."

Ryo gave a small nod, showing he understood. Kagaya Ubuyashiki chuckled and added warmly:

"Think nothing of it. If Mr. Ryo hadn't brought this up himself, I wouldn't have believed it either."

Just then, Ryo—his expression calm and unbothered—spoke up: "Once this meeting's over, Kagaya, please distribute these."

As he spoke, he pulled out fifteen small booklets.

They were detailed manuals on Sun Breathing—meticulously compiled by Ryo himself, complete with his personal research notes.

While Ryo wasn't exactly a master of breathing techniques, the power of the Hōgyoku made that a non-issue.

Right now, even if Yoriichi were resurrected, Ryo's understanding of Sun Breathing would be on par with his.

In fact, with all the refinements Ryo had made, Yoriichi might actually need a year or two just to re-learn his own technique.

"Much appreciated," Kagaya said, taking the booklets and setting them carefully to the side.

Across the room, the Hashira were practically drooling over those little manuals. If the meeting hadn't still been going, they probably would've mobbed him to get a peek at the legendary technique.

That's when Ryo noticed Kanae Kocho winking at him.

He paused for a second, then smiled and gave a nod. She turned to Kagaya and spoke with a cheerful tone:

"Lord Ubuyashiki, I believe there's something else we should present during this meeting."

"Oh?" Kagaya raised an eyebrow. "And what might that be?"

Everyone else glanced over curiously.

Kanae reached into her sleeve and pulled out a notebook, handing it directly to Kagaya.

He opened it and started reading—then froze.

His expression grew more and more serious as he flipped through the pages, eyes widening by the second.

"This... this information... is it real?" he asked, stunned.

"It is," Kanae confirmed, her tone laced with mixed emotions. "That notebook contains intel on all six Upper Kizuki—their abilities, weaknesses, everything."

"WHAT?!"

Rustle, rustle, rustle—in a flash, the other six Hashira swarmed around Kagaya, eyes locked onto the notebook like hawks.

Kagaya cooperated, turning the pages so everyone could see.

"Upper Kizuki One, Kokushibo... He uses Moon Breathing? A direct evolution of Sun Breathing? Wait—he used to be part of the Demon Slayer Corps?! That traitorous bastard!"

"Upper Kizuki Two, Doma... He's just evil."

"Upper Kizuki Three's straightforward enough, but Number Four? That one's tricky. A demon with multiple bodies, all tied to a single core? And the bodies can merge? Damn. One wrong move and it's over."

The notebook wasn't very thick, but it gave them everything they needed. By the end, each Hashira had a solid idea of how to fight every Upper Kizuki.

To them, this intel was even more valuable than Sun Breathing itself.

Sure, Sun Breathing boosted their power—but this information boosted their chances of survival. Some of them were already thinking they could solo an Upper Kizuki now.

The shift in morale was massive.

Everyone turned to Ryo—the man behind it all.

Forget whether or not he could fight. Just this level of intelligence gathering made them respect him deeply. Some were even mentally assigning him labels like "jack of all trades".

At that moment, Giyu Tomioka frowned slightly and asked:

"Do we have any intel on the Lower Kizuki?"

.

.

.