If you side with the Vampires during the quest line, Harkon reveals he was once a mighty king who was very powerful and ruled vast territories, but if I'm not mistaken there isn't any mention of a ruler or a High King in Elder Scroll lore who was named Harkon or of the Volkihar clan. Could he have been some other King during his time as a human only to have changed his alias when he pledged his servitude to Molag Bal? I think that is very possible given Molag Bal's reputation with "Screwing around" with people in a very perverse manner.