Tamayo only allowed herself to breathe once Tsukihiko's figure had completely vanished. A long, shuddering sigh escaped her lips as the tension that had seized her entire body finally began to fade. Every second she had spent in his presence had been an agony unlike any she had felt in centuries.
In her eyes, the Muzan Kibutsuji of the past had been a creature of pure, fanatical evil. But this man, this version of him that called himself Tsukihiko, was something far more terrifying: a rational demon. A demon was already a creature of nightmares, but one that acted with cold, calculated logic was a horror beyond imagination. His immense power was undeniable. Tamayo knew that even with all her strength, she stood no chance against him.
"Demon… demon…" she murmured, her voice trembling.
Just then, Yushiro and Nezuko rushed to her side, drawn by the sound of her distress. They gently helped her to her feet.
"Lady Tamayo, are you alright!?" Yushiro asked, his own body shaking with a mixture of fear and concern. He knew there was only one being in the world who could distress the always calm and gentle Lady Tamayo to such an extent. It wasn't a person. It was a monster. It was Muzan Kibutsuji.
Beside them, Nezuko looked up at Tamayo, her brow furrowed with worry. Though she could no longer speak, her soft hums and the gentle way she held onto Tamayo's sleeve conveyed her deep concern. She could feel the kindness that radiated from this woman, and seeing her in such pain was upsetting.
"I… I'm fine," Tamayo managed to say, forcing herself to regain some of her composure for their sake. But even as she straightened her back, the profound horror still lingered on her face. The word 'terrifying' was not enough to describe the mark Tsukihiko had left on her soul. It was a feeling of pure, heart-stopping dread.
At the same time, Tsukihiko's form materialized back in the Infinity Castle. He moved through the shifting, impossible architecture as if he were walking through his own garden.
"Tamayo is still so easily frightened," he mused to himself, a cold, analytical tone in his thoughts. "Her medical knowledge, however, remains extraordinary. It is a valuable asset."
He considered the role of medicine in his long existence. In the entire world, perhaps only the Demon Slayer Corps' Insect Hashira, Shinobu Kocho, could match Tamayo's expertise. Doctors held a surprisingly powerful position in this world. Three of them, in particular, had played significant roles.
First, there was the foolish doctor who had treated him in his human life. That man had accidentally created the first demon—a perfect being—and was then killed for his incompetence before he could even finish the treatment.
Second was Tamayo herself. A brilliant traitor who, over hundreds of years of independent research, had developed a medicine capable of aging a demon's cells. A remarkable feat. She had even sacrificed herself in a desperate, final attempt to kill him in another timeline. Such dedication was useful, if misguided.
And third, there was Shinobu Kocho. One of the Demon Slayer Corps' nine Hashira and the head of their medical team. She had successfully created a potent poison from Wisteria flowers that could kill demons. She was clever, like an insect that had developed a sharp sting. In the end, she had managed to help kill Upper Rank Two, Doma, alongside her adopted sister.
Tsukihiko smirked. Medicine could create demons, and it could kill them. It was a tool, like any other. And now, all the pieces were in place.
"It's time for a new show to begin," he said, a faint, cruel smile gracing his lips.
Meanwhile, in another corner of the vast Infinity Castle, three figures struggled against the endless, shifting corridors.
"Damn it! What is this place?" a voice snarled in frustration. "We've been walking for what feels like days, and we haven't found anything!"
"If it's an illusion, there seems to be no way to break it," another voice added, calm but laced with strain. "And if it's real, then how do we get out?"
"This is an incredibly bizarre space," a third, older voice observed. "The Blood Demon Art that controls this place is more powerful than anything I have ever encountered."
The three men were the former Water Hashira, Sakonji Urokodaki; the current Water Hashira, Giyu Tomioka; and the Serpent Hashira, Obanai Iguro. They had been trapped here ever since Tsukihiko had captured them. Obanai, whose skills as the Serpent Hashira made him an expert at navigating difficult terrain, wore the grimmest expression of all. The castle seemed to have no discernible pattern, its rooms and hallways rearranging themselves at random.
Worse still, demons constantly emerged from the woodwork, forcing them into endless battles. Though they had defeated every demon they faced, their bodies and minds were nearing their breaking point. Their muscles ached, their lungs burned, and the constant disorientation was wearing down their wills. It was, without a doubt, the most terrifying crisis they had ever faced—a slow, grueling process of being worn down to nothing. It was like being boiled alive.
"If I'm right, this must be the work of one of the Twelve Kizuki," Sakonji Urokodaki said, his voice steady despite their situation. "However, I don't think the demon's goal is to simply exhaust us to death. Its true purpose is to push our bodies and spirits to their absolute limits. Only then… will our real enemy appear."
As if on cue, the moment Urokodaki finished speaking, the space around them warped violently. The familiar corridor dissolved, and the three of them found themselves standing exactly where they had first appeared. Before them, a massive platform rose from the ground, and upon a throne that seemed woven from blood and fire, Tsukihiko's figure materialized.
"Damn you…" Giyu Tomioka and Obanai Iguro instinctively reached for their swords, a sharp killing intent flaring in their eyes. The memory of how this man had casually murdered their friend and fellow Hashira, Sanemi Shinazugawa, was still fresh in their minds. He had crushed the Wind Hashira as easily as one would step on an ant, a horrifying display of the power gap between them.
Yet, he hadn't killed them. That was the most infuriating and unnerving part. From the day they joined the Demon Slayer Corps, they had accepted the possibility of death. They would fight to their last breath if there was even a sliver of hope for victory. But now, that sliver of hope was being held and controlled by their greatest enemy.
"Speak," a calm voice cut through the tension. Sakonji Urokodaki slowly removed his red tengu mask, revealing the face of a gentle but resolute old man. He looked directly at the demon on the throne.
"Muzan Kibutsuji, what is your plan? We are here, and as you wanted, our bodies and our wills have reached their limits." His voice was cold and clear, without a trace of fear. "If you're going to make your move, do it now. We can't hold on much longer."
"Is that so?" Hearing this, the smile on Tsukihiko's face widened into a grin of pure amusement.
"Don't worry," he said, his voice echoing in the vast space. "You can't die just yet. If you did, how would my Infinity Train plan ever begin?"
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