-MC top modern assassin with high completion rate transmigrate to ancient martial world -MC builds hidden assassin organization in the martial world -MC trains young kids how to be an assassin to be members for his organization this is all I can remember. PS. might be one novel or many different novels combined.
Does sound like an Otherworldly Evil Monarch but I can't remember him, training kids. The Nine Cauldrons : is also top assassin reincarnated but I don't think he makes any assassin organization. World Teacher – Other World Style Education & Agent (WN) Is jp isekai WN about another top assassin who reincarnates and he does train kids IIRC but I don't think he makes organization at least not in the first few chapters that I have read years ago.