Here's Demon Slayer's mythical Blue Spider Lily flower explained. Demon Slayer follows hero Tanjiro, who joined the Demon Slayer Corps in hopes of finding a cure for his infected younger sister Nezuko. The anime has proven hugely popular since its 2019 debut, to the point that its movie spin-off became Japan's highest-grossing film of all time in 2020. Demon Slayer season 2 arrived in 2021 and further explored the backstory of its main villain Muzan Kibutsuji.
Muzan (AKA The Demon King) is the world's first demon and leader of the Twelve Kizuki, which consists of the most powerful demons, who live to serve their master. Demon Slayer season 1 left little doubt as to his abilities or ruthlessness, and he has no issue killing humans - who he views as lesser creatures to him anyway - and those fellow demons who disappoint him. Demon Slayer the manga has revealed more about him including the fact Muzan was once human himself and was struck with a terminal illness, seeking a doctor's help.
Muzan isn't a demon for any supernatural reason, but from the medicine, this doctor used to treat his illness. The doctor used an extremely rare flower known as the Blue Spider Lily - which doesn't exist in reality - as a base ingredient in Demon Slayer Muzan's treatment. However, it didn't seem to be having any real effect, leading a distraught Kibutsuj to kill the doctor instead. He soon realized the medicine not only made him healthy and strong but immortal too. This was due to the Blue Spider Lily, but his treatment was incomplete, and he murdered the only man who knew where the flower could be found.
Muzan Is Searching For The Blue Spider Lily Flower
Demon Slayer's chief villain then spent a millennium searching for the mystical flower to little avail. In addition to the medication turning Muzan into the first demon, it also ensured neither he nor other demons could exist in the sunlight. Kibutsuj hoped that by finding the Blue Spider Lily he could reverse this and complete his transformation, becoming truly immortal.
This would obviously be a bad outcome for the human race, and Demon Slayer's - which is getting a season 3 - manga later revealed the reason the Demon King couldn't find the Blue Spider Lily is that it only blooms on rare occasions - and only during the day. Kibutsuj hasn't been able to get his hands on the mystical flower as of yet, and if he does, events on the show and manga are likely to take a very dark turn.