The needle pierced Nezuko's arm, injecting Yōsuke's Demon King blood into her body.
Within her, violent changes began to erupt. The Demon King's blood coursed through her veins, wracking her with unbearable pain.
"Nezuko!" Tanjiro cried out, terrified, as Nezuko screamed in agony.
Before the eyes of both Tanjiro and Rui, Nezuko's body rapidly transformed into that of a woman in her early twenties. Two full demon horns sprouted from her head, flowing vine-like markings spread across her skin, and from her back unfurled a pair of blood-forged wings, delicate and lethal like a butterfly's—resembling Yōsuke's own.
Fully demonized, Nezuko radiated an aura so overwhelming it made Rui's heart quiver.
Rui flicked his fingers, threads whipping forth to strangle Tanjiro and then deal with Nezuko.
"Die!"
But just before the threads could slice into Tanjiro, Nezuko's figure flickered like a phantom. In an instant, she appeared at Tanjiro's side, shredding the threads with the razor edge of her claws.
She lifted the exhausted Tanjiro into her arms, her wings carrying them aloft. Her cold gaze, brimming with murderous intent, locked onto Rui.
"What's with those eyes?" Rui spat. "As if you're the hunter and I'm the prey? To keep interfering with me—how dare you! You've pushed me beyond forgiveness! Here and now, you are the prey! Blood Demon Art—Rotating Thread Prison!"
Enraged beyond control, Rui wove his strongest, hardest threads into a massive vortex-like cocoon and launched it toward Nezuko and Tanjiro circling in the air.
Seeing no sign of Nezuko evading, Tanjiro called out desperately:
"Nezuko! Dodge it!"
But instead, Nezuko grasped Tanjiro's right hand—the hand that still held the broken Nichirin Blade. Her blood poured into the shattered edge, extending it into a full blade of fire. The entire sword burned with blistering crimson flame.
With a beat of her wings, Nezuko shot straight toward Rui. Countless threads disintegrated in the heat before they could even touch the flaming blade.
Rui's eyes widened in disbelief.
"Even my hardest threads—burned away?! Impossible! Again! Blood Demon Art—Rotating Thread Prison!"
Once more, he unleashed a storm of threads.
For a moment, Rui thought he had ensnared her. But then, his vision faltered—the figure he had trapped dissolved into blood. It was nothing but a clone.
A chill ran down Rui's spine. Too late, he felt the sting at his neck.
His hand flew to his throat. A burning slash marked across it, flames searing the wound to stop regeneration.
Nezuko's true body stood behind him.
The technique was none other than Yōsuke's Blood Demon Art—Blood Body Replacement. A clone made of blood to deceive the enemy, while the true body slipped behind. With it, Nezuko had severed Rui's neck.
"This… impossible…" Rui's head tumbled to the ground, his eyes filled with disbelief.
He was supposed to be the hunter—so why had he been slain by his prey?
And as his fading gaze fell upon Tanjiro and Nezuko leaning against each other, a long-numb heart inside him stirred for the first time in years.
A memory—buried, locked away since he became a demon—rose up.
As a human, Rui had been frail and sickly, walking itself a torment, running utterly impossible. His life had been confined to weakness and suffering.
Then Muzan Kibutsuji appeared, offering him health and strength. But in return, Rui could no longer walk in sunlight. He had to eat humans.
His parents did not rejoice at his newfound vitality—they recoiled in horror.
Rui recalled a story he once heard: a father who drowned saving his son, dying without regret. Rui believed that was a beautiful bond, that the father had fulfilled his duty.
Compared to that, Rui judged his own parents as unworthy.
When they tried to kill him, Rui—already a demon—killed them instead.
He could not understand why they wanted to end his life. To him, their love was false, their bond an illusion.
After that, Rui tried to fill the void by creating "family" after "family." Yet the stronger he became, the more his human memories slipped away—until nothing remained.
Now, watching Tanjiro and Nezuko's bond, Rui remembered his mother's dying words:
"I'm sorry… I'm sorry I gave birth to you with such a frail body…"
And his father's sorrowful voice before attempting to kill him:
"It's all right, Rui. I'll die with you."
Only now did Rui realize—his father had wanted to bear the sin together with him.
Tears welled unbidden in Rui's eyes as he gazed at Tanjiro and Nezuko embracing.
"So… the true bond… I severed it with my own hands…"
His headless body staggered forward toward them, arms reaching as if to grasp something forever out of reach. Step by step, until it finally collapsed at their feet.
The Demon King blood inside Nezuko was spent. Her form shrank, regressing into that of a toddler no older than three or four.
Feeling the unbearable grief pouring from Rui's broken body, Tanjiro reached out, laying a hand upon him.
"So warm… like sunlight," Rui thought, trembling. "I remember now… I always wanted to say sorry. I'm sorry. Everything—everything was my fault. Forgive me… though I've killed so many… I'll go to hell for sure. I won't be able to go where Father and Mother are…"
"No," came a gentle voice.
Rui's eyes snapped open in shock. He stood in a space filled with light and warmth.
"Let's go together," his parents said softly. "Even if it's to hell."
They embraced him.
Bathed in their love, Rui's demonic form melted away, and he became once more the child he had been—a boy with human feelings, crushed with guilt, but embraced by forgiveness.
Sobbing, he buried his face in their arms.
"Father, Mother… it's all my fault. I'm sorry, I'm sorry, I'm sorry…"
At last, redeemed, Rui's body dissolved into ash, scattering into the winds of Mount Natagumo.