Demon Slayer season 3 premiere kicks things off with a significant expository dump and Blue Spider Lily, which serves as one of the core motivations for its prime antagonist.
Demon Slayer anime has kicked into its third season with the Swordsmith Village arc, and before Tanjiro embarks on the adventure contending with weapons and new knowlegd, Muzan makes a scintillating entry, tasking his upper rank demons with two things they’ve failed miserably at for a long time.
One of these tasks is to the track down the Ubuyashikis, and the second one is to find Blue Spider Lily, a flower that both bestowed a boon upon his once terminally ill body and a bane that has haunted him for more than a thousand years.
A cure for illness
Before Muzan was the mythical monster that he is currently in the anime, he was a human — a human who had a terminal illness.
There came a doctor, proficient and knowledgeable enough to apply an extremely rare flower, called Blue Spider Lily, to Muzan’s treatment.
Muzan’s impatience and ignorace gets the best of him, and out of frustrations born from not seeing an improvement, he kills the doctor. Soon, however, he comes to regret it most immensely.
He finds that the doctor’s cure worked, and to an extent that renders Muzan immortal. However, his improved health, super strength, and immortality also comes with a price because he doesn’t go through the complete cure; he becomes fatally allergic to sunlight.
Muzan’s millenium-long hunt
Moonlighting as the monstrous first demon of the world, Muzan spends a millenium searching for a cure for his vulnerability against the sun. Meanwhile, he also gives rise to an entire hellscape of demons through his blood.
His demon descendants, though, have the same vulnerability as he does. With the only person who knew where to find Blue Spider Lily gone, Muzan has no way of knowing where to find it.
He repeatedly sends his demons on the hunt for the flower so he can make the cure for his only vulnerablity, granting him true immortality and the world, the worst gift in all of history.
So far, Muzan and his demons have failed to track down the flower, and with every passing day of sunlight that eludes Muzan, he gets more and more enraged and impatient.
In Demon Slayer season 3 premiere, Blue Spider Lily is mentioned by the villain who’s working, and failing, to concoct a solution that can rid him off his vulnerabilities. This sets the stage for the anime to soon delve into his backstory as well as explain why he’s been unable to find it.