Blue Spider Lily

The Blue Spider Lily is a mythical flower, said to look like a Spider Lily with blue colors instead of red. It has been described as an important ingredient of a medicine for Muzan's treatment by his doctor during the Heian Period.

History
The Blue Spider Lily was an important base ingredient for the medicine of the doctor who treated Muzan's deadly disease. It was supposedly able to cure Kibutsuji Muzan's disease that will kill him before he turned twenty. However, frustrated by the worsening condition of his disease, the doctor was killed by Muzan before he could finish the medicine. It was then that Muzan realized the medicine was actually working, as he obtained a strong body, but consequently could not walk into sunlight and craved human flesh due to the medicine being a prototype, turning him into a demon. Muzan deeply regretted killing the doctor, as he then realized that his life saving treatment worked and that he had no idea how to finish his treatment by creating the medicine nor to find the Blue Spider Lily.