Valerica is the wife of the Vampire Lord known as Lord Harkon, leader of the Volkihar Vampire Clan. She has become estranged from him and from the clan due to his leading them into a fanatical crusade to carry out an ancient prophecy, to cancel out the power of the sun. She now resides in the plane of Oblivion known as the Soul Cairn, where she is held captive the the Ideal Masters using an arcane force field restricting her movements.
Like her daughter, Serana (who like herself and Lord Harkon is also a Vampire Lord), she is a Daughter of Coldharbour, giving her vampiric bloodline special arcane powers and a place in dark arcane legend. This mystical quality of their blood is relevant to the prophecy being pursued by Lord Harkon, thus to keep him from accessing the power of their blood Valerica sealed Serana in an underground tomb, resulting in centuries of unconscious captivity.