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 Kibutsuji's treatment by his doctor during the Heian Period.

History
The Blue Spider Lily was an important base ingredient for a medicine created by the doctor who treated Muzan's deadly disease (which would supposedly kill him before he reached the age of twenty). However, frustrated by the worsening condition of his illness, the doctor was killed by Muzan before he could finish the medicine. It was then that Muzan realized the medicine was actually working; he obtained a strong body, but due to the medicine being a prototype, he consequently could not walk into sunlight and craved human flesh, turning him into a Demon. Muzan deeply regretted killing the doctor as he realized his life-saving treatment worked and that he had no idea how to finish his treatment by creating the medicine without the Blue Spider Lily.