User blog comment:Zippertrain85/Harkon's backstory?/@comment-5558140-20130507033734/@comment-5558140-20130509004733

we also don't know enough to say that he's not old enough to be from atmora, serana herself said they pre-date the imperial legion, and he may not have left atmora right when the nords left for skyrim. maybe he stayed behind to become a vampire lord and then moved to the castle after that. i reall think the writers left these things intentionaly vague, either to avoid creating major lore-holes, or to allow players like you and me to speculate on it and formulate our own opinions.