My Patient Was The Demon King: Now So Am I.

Shang Quannai, a jaded modern doctor, reincarnates into Heian-era Japan. His new patient? A frail boy named Ubuyashiki. The horrifying truth: this child is Muzan Kibutsuji, the future Demon King, and Shang Quannai is the doctor fated to create and then be slaughtered by him. With Muzan's blade arcing for his head, Shang Quannai makes a desperate, monstrous gamble. He consumes Muzan's cursed blood, along with a secret Kamado family essence. His body convulses, transforming, rebuilding. He rises from his own blood, no longer human, now a demon himself. Muzan stares, enraged and confused, as Shang Quannai, a new breed of demon, cracks his neck. The true horror isn't dying to the Demon King; it's becoming the one thing he never anticipated.