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Majin? What's that? It doesn't sound at all like a Norse-mythology name, in fact it doesn't sound at all like that! What's this, an error in the matrix? No… I remember there are some Japanese terms mixed around as well, mostly influence of reincarnated people, right? I mean the Winter Lotus Sect was made entirely out of a Japanese freak, Yukihime, who came from another world so everything is possible, reincarnated people can really change this world from its original course, in fact, the existence of this enormous variety of monsters in the world, and that dungeons exist alongside the technology that Greenwood uses to fight all originated from ancient, reincarnated people known as the "Great Demons" or something.
Check "The Protagnist's Sister Is Actually the Strongest"!