Blue Spider Lily

The Blue Spider Lily (い) 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, Muzan killed his doctor before he could finish the medicine. It was then that Muzan realized the medicine actually had effect: he obtained a strong body, but due to the medicine being a prototype, he consequently could not walk into the 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. As a result, he unsuccessfully searched for the flower for centuries in hopes of using it to complete the treatment and give himself the perfect immortal body.

Over a hundred years after Muzan's death, botanist Aoba Hashibira discovered samples of the Blue Spider Lily and figured out that it only blooms at daytime two or three days a year, and when it does, it only blooms for a short period of time and then it closes back up, resembling that of a big horsetail weed. In some cases, depending on the climate, the blue spider lily didn't even bloom at all. This explains why Muzan never found it, as he would have been unable to go out during the day. Unfortunately, due to a mistake, all the samples of the flower he had withered, likely preventing the possibility of the resurrection of the Demon race as we know it.

Trivia

 * Kie Kamado knew where the Spider Lilies bloomed, and showed them to Tanjiro Kamado when he was very young. This is why he was vaguely able to recall them during his later battle with Rui.
 * The Blue Spider Lily is a flower that does not exist in reality. However, the Red Spider Lily is an important flower in Japanese folklore, believing to guide the dead to their next reincarnation. They are also believed to grow upon the shore of the Sanzu River, the river of the dead in Japanese folklore, and all who die must traverse the width of it. It is possible the author intends for the Blue Spider Lily to represent the antithesis of the Red Spider Lily, which would be damnation.