The thick canopy of trees above Tanjiro, Nezuko, Gon, and Killua filtered the light into fractured beams, casting strange shadows over the ruins they had just discovered. The deeper they traveled into the forgotten city, the more the air seemed to hum with an ancient, forgotten energy. Every step forward was met with a creeping sense of being watched—though by what, none of them could say.
The road that led them into this city was long and worn down, crisscrossed with roots and debris. Stone towers loomed in the distance, barely standing but enough to hint at the city's former glory. The group had been silent for most of their journey, each lost in their own thoughts, their senses on high alert after their battle with the shadow entity.
Nezuko, walking closely beside Tanjiro, tugged on his sleeve and pointed toward the large mural in front of them, half hidden by ivy. The mural was massive, spanning the entire wall of a broken-down building, and was still remarkably detailed despite the centuries that had passed.
Tanjiro stopped in his tracks, his breath catching as he took in the sight. "This… this is incredible."
Gon and Killua moved to stand beside him, their eyes locked on the mural. It depicted a scene of ancient warriors standing in formation, their bodies radiating power. Above them, a figure resembling a king or god watched from a throne of clouds, his eyes glowing with a terrible light.
Killua's sharp eyes immediately picked out the details. "Look at their stances," he said quietly. "Those warriors… they're using breathing techniques. But their posture, the way their bodies are drawn… it's different from what we know."
Tanjiro stepped closer, studying the mural. The warriors' chests were expanded, their muscles perfectly sculpted, but there was something different—something passive about their power, as if they didn't need to exert any effort to maintain their strength. Their breathing wasn't an active technique like the one Tanjiro and the other Demon Slayers used; it seemed like it had become part of their very being.
"I think…" Tanjiro began, his voice soft but filled with awe, "this is what breathing techniques were like before the fall of this civilization. They didn't just use them for combat—they lived with it."
Nezuko nodded quietly, her sharp eyes following the story the mural told. As they moved further along the wall, the images became darker, the once-proud warriors now shown in chains, kneeling before the godlike figure.
Killua's eyes narrowed as he examined the next part of the mural. "Something went wrong," he said. "Look here." He pointed to a section of the mural where one of the warriors, a man who seemed to be leading them, was drawn with twisted, darkened features. His eyes were wide with madness, his body grotesquely swollen. It was an unmistakable image—he was transforming into something terrible.
"A demon…" Tanjiro whispered. His heart clenched as he stared at the scene. "That warrior became a demon."
Killua frowned, running his hand over the carvings. "I think this is where it all started. The ancient civilization wasn't destroyed by outside forces—it was destroyed from within. And it looks like the royal family played a big part in it."
Gon, who had been quietly observing, tilted his head. "But why? Why would someone as powerful as this king curse his own people?"
Tanjiro's gaze shifted to the image of the godlike figure at the top of the mural. The glowing eyes, the raised hand—it was a symbol of judgment. "Maybe it wasn't just a curse. Maybe… it was punishment. This civilization must have grown too powerful, and they angered this god."
Nezuko, her expression thoughtful, hummed softly. She pointed to a section of the mural where the same darkened warrior had been chained and placed into what looked like a crypt. His features were still grotesque, but now, other warriors, still in their human forms, stood over him, looking down with expressions of grief.
"This was the start of the demons," Tanjiro said, his voice trembling. "They were the descendants of the royal bloodline—punished for trying to control power they shouldn't have."
Killua stepped back from the mural, his sharp mind already piecing things together. "This makes sense," he said, his voice steady but thoughtful. "If the ancient civilization had mastered breathing techniques to such a degree that it became passive—meaning they didn't have to think about it or focus on it—then their bodies would have undergone long-term changes. Their strength, stamina, and speed would have naturally increased, even when they weren't in battle."
Gon raised an eyebrow. "Like they permanently leveled up?"
Killua nodded. "Exactly. Their breathing techniques would have been like a constant state of heightened physical performance. Over time, the body would adjust to that. But…" He glanced back at the mural. "If they pushed too far—if they tried to go beyond what the body could handle—it could have led to disastrous consequences. Maybe that's why this god stepped in. The royal family might have been trying to take control of something they couldn't."
Tanjiro looked down at his own hands, his thoughts racing. "That would explain why the Demon Slayers today can't use the breathing techniques like this. The knowledge was lost. We're only scratching the surface of what the ancient warriors were capable of."
Nezuko, still observing the mural, pointed to one last section. It showed the warriors kneeling before the god, their heads bowed as they accepted their fate. But one figure—a woman standing behind the king—remained upright, her hand placed over her chest in a gesture of defiance. She didn't look angry, but determined.
Tanjiro's eyes widened. "She's… refusing the curse."
Killua's eyes lit up with understanding. "That must be the key," he said, his voice filled with excitement. "The royal bloodline carried the curse of demons, but there were some who rejected it. That's why not everyone becomes a demon. The ancient breathing techniques must have had the ability to resist the curse, but over time, that knowledge was lost. That woman—she knew the secret."
Gon leaned in, his eyes wide. "So, if we figure out what she did, we could stop the curse for good?"
Tanjiro nodded slowly, his heart pounding. "Maybe… But we're still missing something. I need to learn more about these techniques."
Nezuko, sensing her brother's determination, hummed softly and placed a comforting hand on his arm. They were close to uncovering the truth—closer than ever before.
A Glimpse of Ancient Power
The group continued to explore the ruins, finding more murals and carvings that hinted at the true power of the ancient breathing techniques. In one chamber, they found a series of tablets, each engraved with detailed instructions on how to control one's breathing to reach a passive state of strength—similar to the techniques used by the ancient warriors.
Tanjiro, kneeling before the tablets, studied the symbols with intense focus. "This… it's the key. These tablets are the basics of the true breathing techniques."
Killua, standing beside him, crossed his arms. "If you can figure this out, it could change everything. The modern Demon Slayer Corps wouldn't have to rely on flashy techniques or constant focus in battle—they'd be stronger, all the time."
Tanjiro's brow furrowed as he tried to absorb the knowledge. "I can't master it all, but… I think I can learn enough to help the Corps. It won't be like the ancient warriors, but it'll be a start."
Nezuko, her eyes filled with determination, nodded in agreement. She could sense that they were on the cusp of something important—something that would change the course of their battle against demons forever.
As Tanjiro stood, he felt the weight of the ancient civilization's legacy settling on his shoulders. They had uncovered the truth behind the demons, the curse of the royal bloodline, and the lost power of the true breathing techniques.
But there was still more to discover—and more danger ahead.