Celebrating her big day with boyfriend John Mayer, KatyKat got into character, wearing a sexy white and red vampire costume while John wore a prosthetic nose and went as a tough and rugged zombie hunter.
Katy's transformation into the Queen of the Damned took some work though!
She put on blood red contact lenses and realistic vampire fangs for the party, which was held at the Magic Castle in Hollywood.
Look (below) at the pics to see her complete conversion into a blood-sucking vamp!
[Image via Mark Hunter/The Cobra Snake.]