Serana is a Nord vampire lord and member/former member of the Volkihar vampire clan. She is the daughter of Valerica and Lord Harkon.
Serana and her mother took part in the vile ritual demanded by Molag Bal to become vampire lords. She was sealed away by her mother (without being told why) to keep her from her father after he became abscessed with a prophecy to block out the sun. Lord Harkon was willing to sacrifice his daughter in order to do so, seeing it as a benefit to all vampire kind.