Muzan was ultimately defeated by the collective efforts of Tanjiro, Nezuko, and the other demon slayers. Their teamwork and perseverance were key to ending Muzan's reign of terror in the manga.
Well, muzan x reader fanfiction often explores a very interesting relationship. In such fanfictions, the writer usually dives deep into the dynamic between muzan, who is a complex and often dark character, and the reader - insert, which can be a very exciting exploration of power, mystery, and sometimes a touch of romance. It allows fans to put themselves in a fictional world with muzan and experience different scenarios.
Transformation is another popular theme. It could be the reader - insert changing muzan, or the other way around. Maybe the reader - insert is transformed into a being like muzan, or muzan starts to change his ways due to the influence of the reader - insert. These themes add depth and intrigue to the muzan x reader fanfiction.