Shrouded in mystery while forever eluding the clutches of the Demon King, the Blue Spider Lily is an enigmatic flower that is said to hold the key to curing Muzan Kibutsuji in Koyoharu Gotouge's Demon Slayer. The flower, which blooms at select times of the year, was also the reason behind his transformation into a demon, and till date, all of Muzan's efforts to find it have gone in vain. The premiere of Demon Slayer's third season, which adapts the Swordsmith Village Arc from the manga, displayed the full extent of Muzan's fury towards the Upper Moons, regarding the death of Upper Rank Six Gyutaro and Daki, as well as their collective failure to find the Blue Spider Lily as he had ordered them to. This culminated in an ultimatum he issued to his subordinates to snuff out the recent progress made by the Demon Slayer Corps in turning tide of the battle between humanity and demon kind. In response, Upper Rank Four and Five then set out on a mission to discover and eliminate the residents of the Swordsmith Village, in the process, confronting a force led by Tanjiro, the Love Hashira Mitsuri Kanroji, and the Mist Hashira Muichiro Tokito.
Muzan's desire for the Blue Spider Lily has been central to his goals for centuries now, and the reasons behind this are quite straightforward as it was the source of his current powers. While the flower itself does not exist in reality and is only a part of the lore of Demon Slayer, the Blue Spider Lily is based off the very real Red Spider Lily, which has surfaced prominently throughout Japanese folklore. This flower is said to grow on the shores of the Sanzu River, which is the river of the dead in the Japanese Buddhist tradition, much like the Hindu Vaitarani, and the River Styx from Greek mythology. Red Spider Lillies are said to guide the souls of the death to their next life, and it is likely that Gotouge intends for the Blue Spider Lily to serve as the antithesis of this concept, acting as an instrument of damnation rather than salvation.
The Origins Of Muzan's Ailment
Born with a mysterious terminal illness that all but sentenced him to death at an early age, Muzan's life was fraught with ailments and poor health, and many within his family expected him to die before even reaching the age of 20. Cursing his fortune and resentful of the hand he had been dealt, Muzan developed an intense fear of death as a result of his frailty. Ironically, this small portion of time that he spent as a human would remain in his memory for centuries, despite it being but a sliver of his actual lifespan. Towards the end of this period as his health declined even further, he was subjected to a prototype medication developed by a doctor in the hopes of saving him from his condition. Initially believing the treatment to be ineffective, Muzan killed the doctor in anger and subsequently realized that it had in fact, cured him of most of his woes.
Given an incredibly strong body with the ability to subjugate those around him with his now transformative blood, Muzan assumed the mantle of the Demon King and the progenitor of all demon kind, turning many over to his side to further his ambitions. However, there were a few drawbacks he experienced, most notably that of being unable to go out in the sun. Doing so left him horribly disfigured and burned, turning his body to ash, while also severely hindering his regeneration to a perilous degree. As such, the nigh on immortal Demon King found his one and only weakness was the sun itself — something that he would pass on to all those he held dominion over. On the other hand, Muzan also experienced an intense craving for human flesh, but this was no concern to him as his general lack of empathy allowed him to satiate this need without any real issue.
Why Is The Blue Spider Lily So Valuable?
Killing the doctor that had cured him so hastily was an action that Muzan came to sincerely regret throughout his extremely long life, as his treatment with the experimental medication was still unfinished at the time. Muzan believed that this was the reason behind his inability to go out into the sun, and it ws something that he grew obsessed with. The flower's medicinal properties were also relatively untested at the time he was administered the medicine, and the treatment itself was extremely hard to replicate as Muzan slew the only one who knew of its true composition. It has been heavily implied that the flower can eliminate his weakness to the sun, rendering him truly immortal and invulnerable to any sort of external attack or element. This would finally fulfill his ambition of living without any fear of death, becoming a being a that who could truly be described as eternal.
Another condition for this desire to be satiated was the creation of a demon who could withstand the threat of the sun, who Muzan could use as a means to study his ailment and potentially overcome it. His discovery and eventual murder of the Kamado family at the start of the series was partly motivated by this, evidenced by how he injected certain members of the family with his blood. At first glance, he believed the endeavor to be a complete failure as nearly all the members of the family succumbed to injuries, with one exception. Only Nezuko, Tanjiro's younger sister, survived the fallout of Muzan's attack, and was turned into a demon herself. This would prove to be a major plot point towards the end of the series, when it is discovered that Nezuko is able to overcome the effects of the sun and attain the power that Muzan had so craved for centuries.
How Has Muzan Failed To Attain It?
Unfortunately for Muzan, the Blue Spider Lily was an evasive flower to find, as it only bloomed during daytime at two or three instances each year. In certain climates, the Blue Spider Lily will fail to bloom entirely, making it even more difficult for Muzan to secure the flower. After blooming, the flower closes up and resembles a horsetail weed, losing all the properties that made it so valuable to Muzan. Hence, it was extremely difficult for him to even venture out and seek it, and he eventually delegated the task to his subordinates, who were even less equipped to search for it, by virtue of being vulnerable to the sun themselves. In fact, till date, Kie Kamado, Tanjiro's mother, was shown to be the only one aware of a spot where Blue Spider Lilies bloomed, having taken her son there while he was still quite young. This leaves Tanjiro as the only remaining individual in Demon Slayer known to be aware of where the flower blooms, making him an invaluable asset to Muzan, in the unlikely event that he ever manages to recruit him.