Lady Tamayo unfortunately did not create a cure for Nezuko, instead, it made her gain more abilities and power—you can compare her to Muzan's abilities. The demon king heard about this and planned to make Nezuko his successor. He figured out she's the perfect vessel. He went in hiding for a while to plan everything.
The Kamaboko squad replaced the old generation. The trio rose to the rank of hashiras and became new role models.
As for Nezuko, she still fought alongside her brother. The failed medicine did not affect her love and care for family and friends.
Finally, the faithful day came. Muzan attacked the headquarters, but this time, he was more powerful because he got his hands on the mysterious blue spider lily. The flower that started it all.
They all fought him, risking everything. Nezuko and the trio fought with the demon king with all their might. They managed to land severe blows. The moment Muzan is on the brink of his death, the demon slayers are either exhausted, limbless, wounded, or dead.
The trio is also near their deathbed. Watching her brother apologizing that he can't be with her anymore was the worst thing she experienced.
Muzan has already injected all his blood into Nezuko beforehand. His body died, but his blood is already transferred to the demoness.
Little did he know, the blue spider lily is poison. Let's just say that Muzan's blood is rare despite him being sick. Consuming a small amount of blue spider lily extract can kill a normal person since its a very dangerous poison. It was a miracle that his blood and the extract worked together, thus creating the first demon king.
Even if you're a demon king, consuming another small extract of that flower does not mean power. A temporary power has consequences. It only means suicide.
This is exactly what happened to Muzan. The moment he transferred his blood to Nezuko, Nezuko's blood absorbed his powers. Muzan got burned to his total death when he tried to reside and control Nezuko's body.
However, Nezuko did not notice that Muzan's powers are transferred to her. She just keeps crying. Her brother and friends died before her.
Now that Muzan is defeated and they won, Nezuko is the only demon left in this world. The medicine did not work and her hopes of becoming human again are gone.
"K-kanao." She reached for her brother's lover, who was extremely injured but still standing.
"We won, Nezuko." The butterfly girl said, tears forming in her eyes. She can't accept that her lover is lying dead together with his friends.
"Kill me, Kanao," Nezuko said seriously.
"I-I can't kill my lover's sister!" Kanao's eyes widened.
"Please Kanao. I'm the only demon left. I am a threat, right? I'm begging you." She cried.
Kanao sighed. She grabbed her katana and swung it to her full strength. Nezuko closed her eyes, a single tear fell into her cheeks.
She died. But it wasn't the end of it all.
Nezuko got reborn in a mysterious world, her powers are still there. Her demon curse did not leave her even after death.
