Mukaisen

I’m not sure what “mukaisen” refers to in this context. It does not correspond to a widely recognized topic in English-language reference sources, and it could be a word in Japanese, a name, a fictional concept, or something else. Could you provide a bit more context or specify which sense of “mukaisen” you want the wiki-style article to cover? For example, is it a linguistic term, a place, a person, a work of fiction, a concept in a game or series, or something entirely different? Once I have that, I can generate a concise, neutral article of under 300 words.