Inside the Infinity Castle, the Biwa Demon sat cross-legged on a wooden floor. 'Twing!' Her fingers drummed against the strings as her Blood Demon Art tirelessly searched the location of Ubuyashiki and Nezuko, scanning the countless regions of Japan.
Suddenly, her fingers stopped playing the biwa. She opened her mouth and spoke softly, "Muzan-sama, the Hashira who killed Akaza has arrived in the town."
Muzan, who had been quietly reading a book, closed it and turned his gaze to Nakime.
"Shall I teleport him here?" the Biwa Demon continued. "He's alone, without support. I could easily kill him within the Infinity Castle."
Muzan's demonic eyes flickered as he considered, but after a moment, he shook his head. "No need. Our focus should be on Nezuko and Ubuyashiki. Once we find the demon immune to sunlight, I will deal with him myself. I don't want to alert Ubuyashiki."
Nakime bowed her head, as her fingers once again began to play the strings of her biwa.
----
A few days passed as Ryou searched for the location of the Infinity Castle within the Tokyo region, but he returned to the Nature Manor without any leads.
Somewhere in the streets, a team of Demon Slayers had met their demise. The area was now a scene of horror, with bodies scattered across the street, their severed limbs painting the streets crimson.
Among the fallen, only one remained alive—Kaigaku—kneeling in front of Kokushibo, the Upper Moon One. His eyes trembled in fear as he looked up at the towering figure.
Kokushibo's deep, commanding voice echoed through the night. "Drink this without spilling a drop of blood. If you spill even a single drop, I will kill you."
Kaigaku's hands shook as he accepted the blood, carefully raising it to his lips to drink. As he drank, he felt his throat constricting as the potent demonic power coursed through his body. His veins bulged, and his skin felt as though it was tearing apart from the inside.
As Kaigaku struggled through the demon transformation, he could feel a new, terrifying strength taking over.
He suddenly clutched his chest as black veins snaked across his skin. His once-human eyes turned pitch-black with golden irises, gleaming with malice. Sparks of electricity crackled around him, reacting violently to the transformation.
His teeth elongated into sharp fangs, and his fingernails darkened into claws. With every passing second, his humanity was swallowed, replaced by a hunger that's impossible to ignore.
Then, it stopped. He rose slowly, his breathing calm, as if reborn. A cruel smile curved his lips as he looked down at his hands. He felt overflowing with power, confident that he could defeat ten of his former human self in mere seconds.
Kokushibo watched in silence, his gaze indifferent. "From this moment on, you are one of us. Be grateful for the power you've received and use it to serve him well."
"Yes," responded Kaigaku in respect.
--
A few days passed, and around evening, Ryou and Gyomei arrived at a secluded location deep within the forest. Hidden away, a wooden house stood amidst a sea of wisteria flowers, their blossoms gently swaying in the breeze. Inside the house were Tamayo, Yushiro, and Shinobu.
"Long time no see, Tamayo-san," Ryou greeted with a smile. His gaze then shifted to Shinobu, who looked visibly worn out. "How are you, Shinobu? It seems you've been busy these past few days."
"I'm good, Ryou-san," Shinobu replied. "I'll feel better after some rest, but I couldn't afford to miss this opportunity to defeat Muzan."
Ryou nodded, understanding her. At that moment, Gyomei suddenly asked. "You already know about Tamayo?"
"Yes. I met her during a mission. Since their goal is also to eliminate Muzan, I knew she'd be a valuable ally. I didn't report it to the Demon Slayer Corps because I know how the others will react." Ryou responded.
"I understand!" Gyomei said.
Turning to Tamayo, Ryou asked, "Have you developed the drug?"
"Yes," Tamayo replied softly, "Thanks to Shinobu-san, we developed it. The effect is better than I thought."
She then explained the details of the 'Anti-Kibutsuji Drug'. "It's a unique, potent formula designed specifically to weaken Muzan Kibutsuji. The drug works in stages, but it requires time to take full effect."
Ryou's expression remained calm, and he asked, "How much time are we talking about?"
Tamayo thought for a moment and said, "First, Muzan must absorb the drug. Once he does, the first stage—Human Conversion—will start. He will attempt to neutralize this stage, but once he does, the next phases will activate."
"So, the first stage acts as a bait?" Ryou asked.
"Yes," Tamayo confirmed. "Even if he doesn't manage to nullify it, the other stages will trigger in order. However, the key to this method is ensuring Muzan absorbs the drug.
But it won't be easy to get near him and inject this drug into his body. It's nearly impossible."
Gyomei, who had been silent up until now, spoke up. "Ubuyashiki-sama has already made his arrangements. According to him, Muzan will come to him tonight."
Ryou crossed his arms and said. "Indeed. I noticed several demon eyes monitoring my manor yesterday."
"If you can create a distraction, I have a plan to inject the drug into Muzan. With Yushiro's Blood Demon Art, we can make it work," Tamayo said, her eyes sharp with determination.
"Tamayo-sama..." Yushiro murmured softly, his eyes filled with concern as he glanced at her.
Hearing their conversation, Shinobu had a worried look and looked at Ryou. "So, it's happening tonight... Ryou-san, my sister—"
"Don't worry, Shinobu," Ryou said and reassured her. "Kanae is strong. Even if she faces an Upper Moon, she won't be easily defeated. And as for Muzan... leave him to me. He's my responsibility."