how did muzan become a demon

Muzan Kibutsuji became a demon due to an accidental transformation brought on by an experimental treatment intended to save his life. In the early 900s, Muzan was a human who was dying from a mysterious and terminal illness. A doctor gave him a medicine made from a rare plant called the Blue Spider Lily. Although Muzan initially thought the treatment was ineffective and killed the doctor in a rage, he soon realized that the medicine had cured him, but with side effects: Muzan became unable to tolerate sunlight and developed an insatiable appetite for human flesh, thus transforming into the very first demon. From that point on, he gained immense strength, immortality, and the ability to create other demons by transferring his blood to humans. Muzan's main goal became finding a cure to remove his sunlight vulnerability and achieving perfect, eternal life.