Muzan Kibutsuji is the main antagonist of Demon Slayer: Kimetsu no Yaiba. He is the Demon King, the first of his kind, as well as the progenitor of all other Demons in existence.
A millennium ago, Muzan was turned into a Demon while trying to cure his own terminal illness, and his goal since then was to live without the fear of death and gain true immortality. He attempts to accomplish this by getting rid of his weakness of the sunlight which he is currently doing in 2 ways: finding the Blue Spider Lily or creating more Demons in hopes that one day one would be able to conquer their weakness of the sunlight.
Muzan is also the one responsible for slaughtering the majority of the Kamado family and transforming Nezuko Kamado into a Demon.
Mugen is the 2d graphics engine that allows you to create fighting games in the style of Street Fighter, King of Fighter and others, created by the software house Elecbyte back in 1999, with the C programming language.
Thanks to Mugen you can create your own fighting game style, customizing it 100%, inserting the characters you prefer, the stages related to each character, the entire graphic interface, in short, the entire game is totally customizable.