What's blue spider lily?
blue spider lily
It is a mythical flower, said to look like a fusion of spider and lily, and is of the color blue instead of red. It is an important ingredient that is used in medicine given to Muzan Kibutsuji.
It was an important base ingredient in the medicine, that was used in saving Muzan's life when he was a human. Muzan killed his own doctor when he thought that the medicine didn't work.
What were the effects he suffered from medicine being a prototype ?
couldn't walk into sunlight.
craved human flesh.
turned into a demon.
Muzan deeply regretted killing the doctor as he saved his life
as a result he is still searching for the lily after all these centuries