Radu's mother was an evil witch set within the Subspecies film series. Her true name was never revealed.
At some point the king of the vampires, King Vladislas, was put under the spell of an evil sorceress who desired a child. This child was Radu Vladislas and all the evil of his mother was passed on to him. When the King finally overcame her spell he banished her from his kingdom.
She refers to King Vladislas' second son, Stefan, as the basterd, and expresses joy at the fact he was killed.
When Radu comes to her for help she makes mention of him having been banished, though the reasons are not specified. When he explains his he commands him to kill Michelle Morgan and claim the Bloodstone once more.