Given that the Season 3 finale of "Demon Slayer" is more than twice as long as an average episode of the series, plenty of new information comes to light even after Hantengu has finally been defeated. For instance, Muzan has mentioned in the past that he wants the Upper-Rank demons to find him something called Blue Spider Lily, but we never learn why until the closing moments of Season 3.

As revealed in his backstory, Muzan was once a sickly young man who was born with a strange affliction that not only made him weak but also forced him to stay inside most of the time. As the spiteful Muzan grew increasingly impatient with his doctor, he finally snapped and killed him, only to find that the doctor's latest mystery concoction had cured him at last — by turning him into a demon.

However, Muzan could still not walk in the daylight, despite his enhanced strength and durability, and with the doctor dead, he only knew that Blue Spider Lily was one of the ingredients that had gone into making the formula. This explains both what Muzan's experiments are all about and what his goal has been all along in spreading the demon plague: a perfect cure that will allow him to keep his power but also let him walk in the daylight.