In barely a handful of years, Demon Slayer has become a household name in anime. Complete with quality animation, refreshingly straightforward storytelling and well-crafted characters, the series' success should come as no surprise. That said, another key pillar to the growth of any shonen series is the main villain. And Demon Slayer's Kibutsuji Muzan, the Demon King of the Taisho era, has proven to be more than a match for Kamado Tanjiro and the Demon Slayer Corps.
As the first and therefore precursor to all demons, Kibutsuji possesses powers and experience centuries in the making. Thanks to his incredible, horrifying abilities, he's not only the leader of the Twelve Kizuki -- the 12 most powerful demons -- but the de facto leader of all demon-kind. As the Demon King, he commands such such fear and respect that even saying the name "Kibutsuji" is considered taboo.
Among his myriad of dread-inspiring techniques in Demon Slayer, Kibutsuji Muzan has the ability to place a curse on any demon imbued with his blood. Should any demon he's created say "Kibutsuji" aloud, the curse is activated -- instantly killing them. This is precisely what happened to Susamaru during her battle with Nezuko and Tanjiro. Although she didn't invoke his name maliciously or derisively, the curse was activated all the same, killing Susamaru.
As readers learned from Harry Potter's Voldemort, few things inspire as much fear of a villain as a taboo of their name. Because of Kibutsuji's blood curse, even the most formidable of demons refer to him simply as "that man" whenever they speak of him, partly out of reverence for his tremendous power -- as well as fear of activating his curse.
However, frightening as Kibutsuji's blood curse is, it isn't infallible. There are at least two demons in Demon Slayer who have managed to overcome the curse through sheer willpower: Tamayo and Nezuko. It's true Tamayo possesses incredible medical knowledge, creating an "anti-demon" drug that is later administered to Nezuko, but it's unclear that she overcame Kibutsuji's name curse through any means apart from self-discipline. Additionally, Nezuko demonstrated her own power of will early in the series by refusing to consume human blood even before taking Tamayo's "anti-demon" medicine. It's just this sort of resolve that conquers even the most terrible of Kibutsuji's powers.
Although Kibutsuji's name taboo has been proven to be beatable, it's only ever been done by two -- albeit impressive -- characters thus far in Demon Slayer. This implies Kibutsuji's only real weaknesses are courage and fortitude in those who oppose him. When his enemies aren't afraid, they resolve to find new, unique tactics around his strength and intellect. Thus, Kibutsuji needs his name curse technique -- both to keep the Demon Slayer Corps wary of him and to keep fellow demons from organizing against him.