By Simon Vozick-Levinson
Updated November 08, 2010 at 03:25 PM EST

Irish actress Fiona Shaw (Harry Potter’s Aunt Petunia) has joined the cast of HBO’s True Blood, Deadline reports. Shaw’s character, Marnie, has been described as an underachieving palm reader who is spiritually possessed by an actual witch. She will be a regular cast member in the show’s fourth season next year.