Nezuko’s demon powers have grown strong over Demon Slayer’s run, but how did Nezuko become a demon?
Nezuko became a demon after Muzan Kibutsuji attacked her family. Demon blood entered a wound while she was still alive, so she became a demon instead of passing away from the injury.
A strong will allowed Nezuko to resist demon temptations and not commit to hurting Tanjiro after being found.
The Chapter Behind How Nezuko Became A Demon
Chapter #1 of Demon Slayer begins with Tanjiro leaving his family behind to sell charcoal to a nearby town. On his way back, an old man convinces Tanjiro to shelter within his home for the night. He explains the existence of demons and their dangers, but reassures Tanjiro that demon slayers operate in the area and keep the trouble at bay.
The following morning, Tanjiro awakens and heads back to his home. He notices the strong scent of blood as he nears his residence, seeing family members on the ground. Nezuko is the only one who survived the attack.
You come to learn later that Muzan Kibutsuji is the demon personally responsible for turning Nezuko into a demon.
Tanjiro carries Nezuko as far as he can before her health worsens. Her demon form takes over, and Tanjiro has to fend her off. Giyu, a demon slayer patrolling the area, pushes her off Tanjiro. He asks the demon slayer why his sister turned into a demon.
That’s easy. Demon blood got in her wound, so she transformed into a demon. That is how man-eating demons multiply.
Giyu to Tanjiro | Chapter #1
Able to convince Giyu that Nezuko’s life is worth saving, Tanjiro accepted the demon slayer’s instruction to seek out an old man named Sakonji Urokodaki who lives at the foot of Mount Sagiri. He’ll have a better understanding as to what to do with Nezuko and will provide demon slaying training to Tanjiro.
That’s the answer. Nezuko became a demon after demon-blood entered one of her wounds. Her family was attacked by Muzan, but she was alive while the blood successfully mixed.