Blue Spider Lily

The Blue Spider Lily is a mythical bloom, a spider lily that appears blue rather than the familiar red. It opens only in daylight for two or three days each year before closing again, and depending on the climate it sometimes fails to bloom at all. This rarity, combined with his inability to walk under the sun, explains why Muzan Kibutsuji never located it despite centuries of searching across Japan.

The flower has no real-world counterpart. Its red relative, however, carries weight in Japanese folklore as a guide for the dead toward reincarnation, said to grow along the Sanzu River. The blue variant may be intended to represent salvation as a counterpoint to the red lily's association with damnation.

The plant carries medicinal properties. It was used to treat an unknown illness that would have killed Muzan before he reached twenty. That treatment was still experimental and proved nearly impossible to recreate.

The same medicine triggered his transformation into a demon, granting him a powerful body and altering his blood so he could create and command others of his kind. The cure carried harsh side effects, including a hunger for human flesh and disintegration into ash under sunlight. The flower is implied to be able to remove a demon's vulnerability to the sun, which is why Muzan hunted for it for so long.

The doctor treating the young Muzan used the flower to prepare his cure. Kie Kamado knew of a spot where the lilies bloomed and showed them to a very young Tanjiro Kamado, which is why he dimly recalled them during his fight against Rui. According to supplementary material, the lilies grew where Yoriichi Tsugikuni had buried his wife Uta. The flower debuts in Chapter 67 of the manga and Episode 34 of the anime.