Tonight marked the first large-scale mission of the Demon Slayer Corps, the organization founded by the Ubuyashiki clan to hunt and eradicate demons. Ten elite pairs—twenty warriors in all—had been dispatched across the region. They expected victory. What they received instead was tragedy.
"Twenty... twenty young warriors, all trained by our best masters—wiped out?"
In the Ubuyashiki estate, Ichirō Ubuyashiki's voice trembled as he stared at the messenger kneeling before him. Fujiwara no Ki, seated beside him, listened with grim disbelief.
"…Have you confirmed this report?" Fujiwara asked, his tone grave.
"Yes, Lord Fujiwara. We've verified it repeatedly. None of the ten teams survived."
Ichirō and Fujiwara exchanged a long, silent look. Shock, grief, and fear flickered in both men's eyes.
"…Understood," Ichirō said at last. "Keep this matter contained. Do not allow the news to spread."
"Yes, my lord!" The hidden agent bowed and departed swiftly.
When the doors closed, Ichirō leaned back, face pale. "What do you make of this, Fujiwara? Twenty of our finest… gone."
Fujiwara exhaled slowly. "I underestimated them. The demons' strength is far greater than expected. A pair of swordsmen cannot handle them. From now on, teams must number three or four."
Ichirō frowned. "That would require more trained warriors. We've already stretched our best. It will take time to rebuild."
"There is no need to rush," Fujiwara replied calmly. "At present, our understanding of demons is shallow. We know only that they walk by night, feed on humans, and possess inhuman strength. We do not yet know their weaknesses, their habits, or whether they act in groups. We must gather more intelligence before committing more lives."
He paused, then added, "Also—send scouts to the villages where demon attacks were reported. Speak to those who lost family. The bereaved may offer insight—and perhaps, with proper guidance, could join our cause. Hatred can be a powerful weapon."
Ichirō nodded solemnly. "A grim idea, but a sound one. I'll see it done."
Thus ended the Corps' first campaign—a complete annihilation. Yet it was only the beginning of the long, bitter war between demon and slayer.
In the months that followed, the Ubuyashiki's hidden network grew stronger. They learned that demons were territorial, each one treating its hunting ground as its domain. They did not cooperate; if one entered another's territory, they fought savagely. This discovery gave the Corps a small advantage.
The following year, new teams were formed—groups of three and four. This time, reports of victory began to reach the Ubuyashiki mansion. Though many still perished, the first successful hunts restored hope to the organization.
From those battles, they learned more about their enemy.
Demons possessed terrifying regenerative power. Ordinary wounds barely hindered them, though the process consumed their strength. More grievous injuries—such as the loss of a limb—greatly weakened them.
Their greatest weakness lay in the neck. Decapitation rendered them powerless, and exposure to sunlight would annihilate them entirely.
With every encounter, the Demon Slayer Corps grew wiser—though at terrible cost. The dead paved the road to knowledge. Even so, the Corps remained at a disadvantage. Demon strength in single combat far outmatched any human, and their regeneration made them near-immortal. Were it not for their solitary nature, humanity's losses would have been catastrophic.
Meanwhile, deep within the shadows of Kyoto, Kibutsuji Muzan, the progenitor of demons, observed all in silence. The mortals' fledgling "Corps" amused him. He had heard of their existence through whispers among his children, but their efforts were beneath his concern.
Let them fight, he thought. Every death, every clash would teach him more about the limits of his creations.
And tonight, Muzan had other plans.
"I've finally found one who fits my purpose."
From the second floor of a quiet home, Muzan gazed through the window at the house opposite. Within lived a modest family of three. The woman of the household, refined and beautiful, carried herself with a dignified calm.
Her name was Tamayo—a physician's wife, descended from a long line of healers renowned for their rare knowledge of medicine and herbs. Muzan had learned about her through discreet inquiries and subtle manipulation of the townsfolk.
Tamayo was known for her intellect and grace, but also for her misfortune. She suffered from a wasting illness no doctor could cure, worsened by the birth of her child. Yet she refused to give up, pursuing treatments and remedies with quiet desperation.
Muzan's expression curved into a faint smile.
He needed what she possessed: the deep medical wisdom of her lineage—their rare remedies, the hidden formulas passed down through generations and never written in public records.
Immortal though he was, Muzan still sought perfection—an end to the one flaw that haunted him: his weakness before the sun. Though he doubted any mortal science could conquer that boundary, knowledge itself was a weapon worth taking.
And so, as the night deepened, the Demon King set his sights upon the woman who would one day defy him.