The flower in the wooden box was the very last Blue Spider Lily in existence. Nightingale had not only picked this one, but destroyed all the others as well.
In other words, the flower before Muzan was his only hope for immortality.
At this realization, the ferocity in Muzan's aura finally burst free.
"...Give it to me."
Summoning the last vestiges of his reason, Muzan restrained himself from simply snatching the flower by force.
"Hand it over, and I'll take all the demons and leave the Demon Slayer Corps alone. I'll even kill every demon but myself. Isn't that a good deal?"
The only reason Muzan created so many demons was to help him find the Blue Spider Lily. But now that he knew only he could obtain it, his underlings were useless anyway. With or without Nightingale's promise, he'd have killed them all sooner or later.
He alone was to become the perfect being—no one else.
Nightingale gazed at the flower in the box, silent, seemingly hesitating.
After a long pause, she tossed the box to Muzan, letting it fall into his hands.
Muzan was so stunned, he barely registered what had happened.
A flood of memories surged through him—his entire life, a thousand years of searching, waiting, agony...
"Finally! At last!"
Exhilaration blazed through every cell, burning away his last shred of restraint.
"Hehehe... hahahahaha!!"
His wild laughter nearly tore the night apart.
Without another thought, Muzan devoured the Blue Spider Lily—he'd lost all patience, couldn't wait even a second longer.
"Yes, yes... I can feel it, the power of the Blue Spider Lily surging in me, transforming my body... Finally, I will become the most perfect being on this earth!"
Excitement and joy exploded in his chest, so intense that his human form almost slipped; the body he wore was always a meticulous cellular disguise, but now he had no need for it.
Even from a distance, Nightingale could tell something was changing in Muzan.
His very existence was evolving... Like a Digimon about to evolve into its ultimate form.
If nothing went wrong, Muzan really might become the immortal, indestructible ultimate lifeform.
If nothing went wrong...
Splurt!
Black-red blood gushed to the ground, pooling beneath him.
From his eyes, ears, nose, mouth—even his pores—blood poured endlessly.
His body convulsed, grotesque veins bulging everywhere, his red plum eyes wide with shock.
Now, he looked every bit the "demon" Nightingale imagined.
His lips trembled. A hoarse, furious voice squeezed from his throat, full of unwillingness and rage.
"This... the Blue Spider Lily... what's wrong with it?!"
"That was the real Blue Spider Lily."
Nightingale's calm voice rang out. Muzan jerked his head up, gaze overflowing with murderous intent—but Nightingale's face and tone were utterly unruffled.
"But Tamayo and I added something extra—a deadly poison prepared just for you. Unless you can fully neutralize it, you won't be able to absorb the flower."
Watching Muzan's body rot, Nightingale recalled a recent conversation...
"Let Muzan devour me."
It was the look of someone utterly resolved to die.
Tamayo's red-tinged eyes were unwavering as she covered her heart, staring at Nightingale.
"Ever since I escaped Muzan's control, he's wanted me dead. He can't kill my cells like the other demons, so the surest way is to swallow me whole. I'll carry the poison with me; when he devours me, he'll absorb it too... My death will be your chance to destroy him!"
Tamayo made the same choice as in the original—dragging Muzan down to hell with her.
After killing her husband and eating her child, Tamayo had lived ready to die. Only her hatred for Muzan kept her going; she wouldn't allow herself to die until she'd taken him to hell as well. She didn't deserve to see her family in the afterlife—not yet.
Faced with Tamayo's resolve, Nightingale was at a loss for words.
As a doctor, she wanted Tamayo to live, to break free from the past, to find happiness again.
But sometimes, letting someone live is crueler than letting them die.
Living on meant Tamayo would forever bear the guilt of killing her beloved husband, of eating her own child. Nightingale could only guess how many times, over the centuries, Tamayo had woken up from nightmares of their corpses.
"There's no need for that. I have another plan."
Nightingale had explained her idea—to hide the poison inside the Blue Spider Lily. Tamayo, however, strongly objected.
"That's too risky! Do you realize how dangerous this is? What if our drug fails? If Muzan absorbs the flower, we'd lose even the faintest hope of killing him!"
Tamayo's face twisted in anguish. She could not accept that such a monster might go on living, shameless as ever.
"We'll do as I said. I'll drag Muzan down to hell myself. That's the safest way!"
"I said, that's not necessary!"
Ignoring Tamayo's pleading, Nightingale's resolve was even harder than hers.
"No one will die in this fight—no one needs to sacrifice themselves. Why? Because... Muzan doesn't deserve it!"
"I can't say if I should stop your suicide, but you mustn't die here—not in front of me, and not for this reason. Muzan should die alone, in hell, with no one to keep him company—just a pitiful, solitary end."
Tamayo had killed, so she would go to hell herself, even if she'd killed for reasons beyond her control. But after months spent working together, Nightingale couldn't bear to let someone so like-minded share Muzan's fate.
"Trust me. This time, Muzan will die. There won't be any escape. And if, by some chance, he does survive, I'll take responsibility and kill him myself."
"If he truly overcomes the sun, then I'll burn him with a fire even brighter than sunlight—until not a single cell remains."
Nightingale's eyes were so resolute, so confident, that Tamayo found herself unable to argue.
...
Though she didn't know the details, Tamayo ultimately chose to trust Nightingale.
Afterward, Nightingale thought she'd managed to stop or at least delay Tamayo's suicide—and that maybe Yushirou would finally be grateful, or at least polite. But no—his attitude was as awful as ever.
Guess I was too naïve...
Still, the current state of affairs showed Tamayo's worst-case scenario hadn't happened.
Their plan had worked: Muzan couldn't absorb the Blue Spider Lily. He'd have to waste time breaking down the poison, unable to conquer sunlight or become the perfect being.
Muzan said nothing, too enraged to speak, his reason overwhelmed. He simply glared at Nightingale, eyes full of murder, panting raggedly.
"I told you, Kibutsuji Muzan."
Nightingale drew her machine gun from her travel bag, her red eyes glittering with cold light.
"Tonight, you die! As the plague you are, you don't deserve to live in this world!"
Muzan's eyes bulged, but his body wouldn't respond. Moments ago, he could control his cells with godlike precision—now, even moving a finger was a struggle. It felt like every cell in his body had turned against him.
Nightingale pulled the trigger. Emerald fire flashed from the muzzle.
In that instant, Muzan roared in fury.
"NAKIME!!"
The drugged bullets tore away two-thirds of his flesh, but Muzan clung to life.
As he roared, a wooden door appeared beneath him. It swung open, and he fell through—barely escaping death.
Unlike the lesser demons who died when their heads were cut off, Muzan had five brains and seven hearts. Other than sunlight or burning out his life force, the only way to kill him was to destroy all twelve at once—which Nightingale had almost done.
But that sudden door had saved him: she managed to destroy only ten of the twelve, leaving one brain and one heart.
Nightingale instantly understood: Nakime's Blood Demon Art—this was the entrance to Infinity Castle.
Without hesitation, Nightingale dove after him, slipping through the door just before it closed.
Falling, falling—gravity dragging her down.
There was no visible ground below, no moonlit sky above.
Her vision spun. She found herself in a bizarre realm, buildings tossed haphazardly in all directions—a world with no up or down.
And not far away, also tumbling downward, was Muzan—now barely human in form.
The instant their eyes met, Nightingale felt a wave of burning hatred.
No more running! Time for the cure!
Now was the perfect opportunity—Muzan, poisoned by the Blue Spider Lily, couldn't self-destruct or regenerate.
Even the Hashira, without Marks or the Transparent World, could probably win against him now!
Nightingale fired again, bullets slicing through the air toward Muzan.
Muzan instinctively tried to transform his flesh into whips and bat away the bullets—but his body only twitched weakly. Between the ravaging poison and his wounds, he vomited more black-red blood.
I don't want to die... I can't die here... Not now, when I'm so close...!
But he was so weak, his body so ruined, that he was thrown back to memories of his own frail, human days.
Suddenly, doors all around swung open.
Like a bursting dam, a tide of hideous demons spilled out, forming a wall of flesh between Nightingale and Muzan.
All of them had lost their human shapes—centipede bodies, spider heads, some with grotesquely swollen limbs... all manner of monstrous forms.
Nightingale's special bullets tore through dozens, but the flood only grew, quickly blocking every path.
There seemed no end to their numbers—and, even more frightening, each was as strong as a Lower Moon.
"So that's it—by pumping them full of demon blood and seeing who survives, he can create an army of Lower Moon–level demons as long as they don't explode..."
As she pondered, the monsters lunged at her, roaring in Muzan's voice: Kill this woman at all costs!
Nightingale's eyes flashed.
Plenty of ammo left in the ampoule drum... Come on, then, viruses! Let's get to work!
"I will wipe away all that is toxic and crush anything that is harmful!"
"...The Angel Cries, and Demons Fade Into Shadow!!"
"[Assault Medicine Full Burst Party]!"