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Interesting read so far but I have one major gripe: MC is the author of the novel that he got isekai'd into, but, yet, he doesn't know anything about the demonic world he created or barely anything about his almost-ultimate villain?? Did he not flesh-out his villain or his motives?? Yeah, the author-MC, later clarifies he concentrated more on the heroes, but this is still your story and your villain is half-demon, so you don't know anything??
Now to the half-demon MC: I'm guessing he's between 16-18 years old. Grew up as a noble (??) in the human world judging from his home, so he didn't have any loyal servants to help him with his escape or to escape with him? Not one?? Did they all betray him?? Sad.
But now he's in the Demon Realm posing as a guard -- does he not have any demonic family? I know an ancestor of his committed treason and sided with the humans, but does that mean the whole family decided betrayal was the way to go?? I think it would be interesting to see if the MC had any demon family left and their reaction to seeing him.