I'm not sure if there is such specific 'muzan x tanjiro mpreg fanfiction' available. Fanfiction can cover a wide range of themes and pairings, but this one might be a rather niche or even against the general tone of the original work in some ways. You could try searching on some large fanfiction platforms, but don't expect to find it easily as it may not be very common or might be against the platform's rules if it contains inappropriate content.
Yes, there are quite a few. Some focus on an alternate universe where they have to team up for some strange reason. For example, in one fanfic, they have to face a new common enemy that is even more powerful than Muzan. Tanjiro has to put aside his hatred for a while to deal with this new threat.
The existence of'muzan x tanjiro mpreg fanfiction' can be attributed to several factors. Firstly, fans often have a deep emotional connection to the characters and want to explore all possible relationships, no matter how strange they seem. Secondly, the fanfiction world allows for a great deal of creative freedom. Fans can take the basic elements of the original story and twist them in any way they like. For this particular type of fanfiction, it might also be about exploring the contrast between muzan, who is often seen as a powerful and malevolent force, and tanjiro, who is more of a heroic and pure - hearted character. By creating this mpreg scenario, fans are able to delve into complex emotional and physical relationships that are far removed from the original plot, but which offer a new and unique perspective on the characters.
You can try some well - known fanfic websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, and you can easily search for tanjiro x muzan fanfic there. Just use the search bar and type in the relevant tags.
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.