Mt. Natagumo

Zenitsu clutched the base of a tree, his legs shaking as he peered into the ominous darkness of Natagumo Mountain's forest. "I'm not going in there," he whimpered, his voice filled with terror. "Did you see how they dragged one of us demon slayers in? No way! Nope, I'm out! You guys can go be heroes, but I'm staying right here."

Bell rolled his eyes, while Tanjiro and Inosuke were already preparing to head in. "Fine," Bell sighed. "But you might change your mind if someone catches you hiding out here."

As Tanjiro and Bell stepped forward, Inosuke pushed past them, bristling with excitement. "Outta my way! I'm going to kill everything that moves in there!" He looked back over his shoulder at Zenitsu, sneering. "Scaredy-cat!"

Zenitsu flinched, gritting his teeth and muttering about how he didn't care what they thought. Yet, as soon as he remembered that Nezuko was somewhere in that forest in her box, he shook himself, an oddly determined expression crossing his face. With a loud gulp, he steeled his nerves and dashed after them, driven by a strange mix of panic and admiration.

The forest was crawling with eerie silence, broken only by the occasional scuffle of leaves or far-off scream. Bell, advancing silently through the trees, felt an ominous shift in the air as he sensed a powerful presence nearby. Suddenly, the massive form of the Father Demon lumbered out of the shadows, towering over him with a grotesque mix of muscle and raw power.

Bell wasted no time. He unsheathed his sword, launching forward with a powerful arc that cleaved through the Father Demon's thick arm. The beast let out a guttural roar, but Bell moved with blinding speed, his blade slicing through with unrelenting precision. In mere moments, the towering demon lay defeated, reduced to nothing more than a bloodied pile of flesh on the forest floor. Bell cleaned his blade, casting a disinterested glance at the remains before moving on.

Meanwhile, Tanjiro and Inosuke encountered the Mother Demon in a desperate clash. Tanjiro, noticing Inosuke's overwhelming strength and willingness to dive into any fight, found an opening to strike at her with pinpoint precision. As the Mother Demon's body began to disintegrate, her expression softened in a tragic acceptance of her fate.

Before they could even catch their breath, they encountered another demon slayer, Murata, who wore an expression of utter desperation. "You're not Hashira," he snapped, trying to mask his fear with disdain. "I'll wait for real demon slayers to come!"

Inosuke glared at him with an unmasked fury. "Coward!" he spat. "You think someone else is going to save you? Pathetic!"

With the path clear, Tanjiro and Inosuke pressed deeper into the forest.

As Zenitsu raced through the trees, his resolve weakened with each step. His mind was swirling with anxiety, but the thought of Nezuko kept him going. "Nezuko, I'm coming to save you!" he muttered, the sentiment swiftly replaced by a shriek as a spider-like demon emerged from the shadows.

The fight was chaotic, with Zenitsu's usual mix of fear and fervor making him both unpredictable and determined. Between frantic strikes, he'd occasionally pause to mutter something embarrassing about Nezuko, only to remember he was in mortal danger and leap back into action. His erratic rhythm, combined with his instinctive thunder-based breathing technique, kept him just ahead of the demon's attacks, but he'd need to summon all his courage to finish it off.

While the battles raged on, Shinobu and Giyuu received orders to assist the slayers on Natagumo Mountain. The Master had sensed a deeper purpose in these events, and while he kept his intentions private, he looked forward to introducing Nezuko to the Hashira soon. To Shinobu and Giyuu, however, this mission was simply about supporting their comrades in eliminating the threats on Natagumo.

Upon arrival, they found Bell seated on a rock, looking entirely unperturbed. He offered them a quick report on the events so far. "Father, Mother, and a few others are down," he said calmly, gesturing to the forest where Tanjiro and Inosuke were locked in combat with the demon they now knew as Lower Moon Five—Rui.

The two Pillars shared a glance, clearly impressed by Bell's efficient report and the sheer number of kills he'd already confirmed. Bell called out to the two fighters. "Tanjiro! Inosuke! Make it a good show, or I'll end it myself. We have an audience, you know."

With the pressure on and the presence of two Hashira watching, Tanjiro and Inosuke fought with renewed energy and skill. They moved in harmony, Tanjiro's precision balancing Inosuke's unrestrained ferocity. The battle reached its peak as Tanjiro unleashed a powerful Sun Breathing form he'd been practicing, its fire-like arcs cutting into Rui's threads and tearing through his defenses. Together, they dispatched Rui in a stunning display of skill and strength.

As Rui's body turned to ash, Giyuu and Shinobu approached the fighters, intrigued by the unfamiliar yet powerful technique Tanjiro had used. "This style… it's different from anything I've seen," Shinobu noted, eyes wide.

Bell explained, "It's called Sun Breathing. Urokodaki mentioned it might be the original, strongest breathing style. It's not easy to master, but it's worth the effort."

Just then, the caw of the Kasugai crows pierced the air. "Capture Kamado Tanjiro and the demon with him. Bring them to Headquarters—alive and unharmed."

Without hesitation, Bell and Tanjiro began the journey back with Nezuko, where the Corps awaited their arrival.

At the Demon Slayer Corps Headquarters, they were greeted by the Hashira. The gathering was tense, with many questioning the presence of a demon among them. However, Bell and Tanjiro held their ground, recounting the battle with Muzan Kibutsuji and the terrifying abilities he possessed.

"He can manipulate blood and bones to create devastating attacks," Bell explained. "We've seen him use whips made from his own body, shockwaves that stun or even paralyze, and his transformation amplifies his speed and power."

Tanjiro added, "He also has immunity to certain poisons and can adapt quickly. If we're to defeat him, we'll need to know his abilities inside and out."

The weight of Muzan's power cast a heavy silence over the Hashira, but Bell's voice cut through it with determination. "We've all seen death, and we've all fought demons, but Muzan represents something bigger. We're here to protect those weaker than ourselves, no matter what it takes. So, we fight."

The words rekindled the fire in each Hashira's eyes. With renewed purpose, they turned their focus to training the next generation to be stronger and more resilient than ever before.

Zenitsu and Inosuke, meanwhile, were placed under intense rehabilitation training. Zenitsu's protests echoed across the courtyard, while Inosuke took the training as another battle, his competitive nature unrelenting. Tanjiro and Bell, however, received specialized training from the Hashira. Sanemi drilled them on relentless, high-intensity attacks, while Gyomei taught them to strengthen their bodies to withstand the most punishing encounters. Giyuu's lessons on seamless transitions between forms added fluidity to their movements, though Tanjiro found the training difficult. Bell, on the other hand, seemed to relish the challenges, throwing himself into every drill with boundless enthusiasm.

As the sun dipped below the horizon one evening, Rengoku, the Flame Hashira, approached Bell with a broad, welcoming smile. "Bell, I've been watching your progress, and I'd like you to accompany me on the Mugen Train mission. If you accept, you'll be my Tsuguko—my successor."

Bell, honored by the offer, nodded eagerly. Before leaving, he turned to Tanjiro with a warm smile. "If I don't return in two weeks, come find us, Tanjiro."

With that, Bell and Rengoku set off for the Mugen Train, the promise of a new journey stretching out before them. Tanjiro watched them go, the weight of his friend's parting words lingering as he steeled himself for the trials to come.