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A doctor reincarnates as a child (random xtra character) into a murim novel he had read prior to death
Main character dies and wakes up in novel he's read, but not the whole series.
Human child was brought to demon world by high rank demon to be his grandchild after parents basically sold him to the devil. He now has to navigate demon school without anyone finding out his secret.
Kami-sama No Iutoori (moegi Yuu)