It was all a plan that Charles and Kagaya Ubuyashiki prepared for Muzan.
Tamayo made two kinds of medicine: the first let Nezuko successfully overcome sunlight, and the second would let her break down the demon blood in her body and fully turn back into a human.
Before taking the second medicine, the last thing Nezuko did was to put on a display of having overcome sunlight in front of Enmu.
Kagaya Ubuyashiki understood Muzan's fear of death very well; if it were only Nezuko, he might not take the bait.
He originally wanted to use himself as bait as well, but Charles talked him out of it. Charles then proposed a new idea, which Kagaya refined into the current operation plan.
With two options set before Muzan, the thought was that once he noticed the danger in one, he would overlook what might be hidden in the other.
To make Muzan believe the Corps had concentrated all its strength into a trap around Nezuko, Kagaya actually summoned all the senior slayers of the Corps.
As long as Muzan sent anyone to take a quick look, they would find the Corps truly gathered together, and he would naturally feel safe going to the other location.
What he didn't know was that Charles and Kagaya never intended to have those people take part in the final battle; the ones who would actually fight were at the other decoy site.
The unexpected twist was that Kaigaku took the initiative to betray the Corps and became a demon. Charles taking Zenitsu instead of him drove Kaigaku into a jealous rage that cost him his reason.
Just as when he was a child he drove away the children who had cast him out, luring demons to eat them in order to save himself and take revenge, Kaigaku merely made what he thought was the most advantageous choice.
But this also accidentally worked in their favor: Kaigaku carried over exactly the message they wanted Muzan to know.
Muzan was a suspicious man, and precisely because of that he trusted his own judgment all the more.
Charles also wasn't worried that Muzan would casually send just any demon here.
With the Twelve Kizuki, Upper Ranks included, being cut down one after another, Muzan had lost trust in the other demons.
Even Upper Rank One, Kokushibo, was no exception; Kokushibo had once reported to him that all who knew Sun Breathing had been wiped out and its lineage had been severed.
Yet someone using Sun Breathing had appeared alive right before his eyes and even slain two Upper Ranks.
Muzan certainly didn't think Kokushibo had betrayed him, but that he had failed in his duties was beyond doubt.
And when it came to the Blue Spider Lily, Muzan would absolutely not allow any mishaps. Suspicious as he was, how could he entrust it to anyone else?
Thus Muzan was bound to step into this trap.
Sure enough, just after night fell on the day the plan began, a man in a patterned suit arrived in this mountain forest.
His light steps and faintly trembling body alike showed how excited and elated Muzan Kibutsuji was.
Muzan's pace slowed, for he had seen that swaying blue blossom.
Faced with the thing he had dreamed of for a thousand years, Muzan's eyes turned rapt. He didn't notice at all the extensive signs of earth having been turned over nearby.
With only the flower in his eyes, Muzan took several minutes to cover the scant dozen meters, until it was within reach.
Worthy indeed of being what could evolve him—the shape of the petals, the color, the scent… hmm… wait, the scent?
Muzan suddenly caught a strange smell—not a natural scent a plant would have, but the acrid tang of gunpowder.
His catlike vertical pupils contracted to pinpoints, and before he could react—
Boom!
A violent explosion shattered the forest, and the blast tore Muzan's body apart in an instant.
Worst of all, the Corps' explosives were packed with a large number of four-sided spikes refined by Charles, functioning like miniature Red Blades; driven into Muzan's body, they inflicted considerable secondary damage.
Out of dust and smoke, a broken figure stood.
Muzan's body was little more than a spine studded with glowing red spikes and half a rib cage, with ragged flesh hanging from it.
Only half his head remained, the eyeball wobbling on exposed socket bone—there was no trace of the gentlemanly figure from moments before.
Charles had improved the trap this time; its destructive power was far greater than in the original course of events.
But Charles hadn't intended to blow Muzan to death outright. This wasn't a game; the boss could just skip out.
Trying to finish Muzan easily would very likely let him get away; in terms of staying alive, Muzan was second to none.
Muzan, realizing he'd fallen into a trap, moved to have Nakime pull him into the Infinity Castle the moment his body began to regenerate.
But just then, someone appeared before his eyes.
Tamayo tore away the concealment talisman and thrust a syringe into Muzan's just-healed neck.
Hatred made the veins at the corners of her eyes stand out. In a tone unlike her usual gentleness, tinged with mockery, Tamayo said, "Seeing you so pathetic is truly satisfying—but what will really drive you to despair is still to come!"
Muzan reached out with his restored hand and seized Tamayo's head, his fingers digging into her eye sockets.
"Tamayo, how are you here? What did you do to me?"
Tamayo smiled derisively. "I'm here to take my revenge, of course! What I just put into your body is a medicine that can turn a demon back into a human!"
Muzan found it laughable; Tamayo was the one who had eaten her husband and child, so seeking revenge on him made no sense at all.
What concerned him most now was the medicine he had been injected with.
"Impossible. You could never make such a drug!"
Tamayo said coldly, "Before, of course not. But now, it's different. Muzan, your fate ends today."
Muzan's expression shifted; he could already see several people closing in.
For fear of spooking Muzan, and of friendly fire, the others had been hiding at a distance.
Feeling the drug begin to take effect in his body, Muzan actually grinned hideously and shouted, "You think this is enough to drive me into a corner? How naive. Where you're going next is true hell.
Just as well—tonight I'll wipe out all you eyesore demon hunters!"
As he spoke, the blasted-open ground suddenly turned into tiered architecture, and a sliding door opened beneath everyone's feet.
Muzan, clutching Tamayo, and all of the Demon Slayer Corps fell through.
Because Muzan had once fled abnormally at the foot of Tanjiro's mountain home, in order to keep him from sensing danger again, Charles and Stella were hiding even farther away and hadn't even extended their perception.
So even when Charles and Stella rushed over at top speed, all they saw was the eerie architecture closing and fading away.
Charles sighed. Kagaya had predicted this possibility when they drafted the plan. His foresight was as uncanny as ever.
The key to the final battle lay in whether those who had been drawn into the Infinity Castle could seize control of Nakime and drag the castle up to the surface.
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