Board Thread:Lore Discussion/@comment-5701011-20140725171950/@comment-6881629-20160122185046

Harkon feared death so made a pact with Molag Bal making him and his family pure-blood vampires, the Grey Prince was (somehow) born normally from a vampire and mortal. He didn't know anything about the vampire part (and living that long he would have probably thought something was up) until you tell him in his quest.