Board Thread:Lore Discussion/@comment-3539474-20130910213054/@comment-17114085-20130921220959

I think its unlikely that Harkon is the Jarl's son, Harkon became a vampire when his wife and daughter were "dominated" by Molag Bal, turning them into "Daughters of Coldharbor". Serana mentions they were part of a cult that worshiped Molag Bal. Its not impossible that he was the Jarl's son its just unlikely.