Blue Spider Lily

The Blue Spider Lily (い) is a mythical flower. It is an important ingredient of a medicine for Muzan Kibutsuji's treatment and the reason he turned into a demon.

Overview
The Blue Spider Lily is a mythical and enigmatic flower that is described to be a spider lily that is blue in color instead of red. The flower only blooms at daytime for two to three days a year, after which it closes back up, resembling a large horsetail weed. In some cases, depending on the climate, the Blue Spider Lily doesn't even bloom at all. This explains why Muzan Kibutsuji never found it despite searching all over Japan for centuries: he was hindered by his inability to go out during the day.

This flower has medicinal properties, being used to cure Muzan of an unknown illness that would've caused him to die before turning 20, albeit the medicine was in its experimental stage and thus extremely difficult to replicate. The Blue Spider Lily medicine was also the cause behind Muzan's demonification, granting him a strong body and changing his blood to have transformative properties, enabling Muzan to create more of his kind as well as hold dominion over them. However, this medicine also had the side effect of causing Muzan to disintegrate into ash if he was exposed to sunlight as well as a craving for human flesh. It is also implied that the Blue Spider Lily can nullify a demon's weakness to the sun if it’s consumed, which is the reason Muzan was spending centuries searching for the flower.

Trivia

 * Kie Kamado knew where the Blue 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 in chapter 39.
 * After Uta's death, the Blue Spider Lilies started growing on her grave.
 * 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 (salvation), which would be damnation.