In the "Demon Slayer: Kimetsu no Yaiba" series, Tanjiro Kamado turns into a demon due to the events that transpire during the battle with the Upper Rank 3 Demon, Akaza.

During their intense fight, Tanjiro sustains severe injuries and is on the verge of death. In a desperate attempt to save him, his demon sister Nezuko offers her blood to him, allowing him to drink it. Nezuko's blood, being that of a demon, possesses transformative properties.

As a result of drinking Nezuko's blood, Tanjiro undergoes a transformation into a demon himself. This transformation grants him enhanced strength, agility, and regenerative abilities. However, Tanjiro's strong will and his desire to protect others allow him to maintain his human consciousness despite becoming a demon.

Tanjiro's transformation into a demon becomes a significant turning point in the story, as he must now grapple with his newfound nature while continuing his quest to defeat the demons and find a way to restore his humanity.