Antoinette Bower played Sylvia, the alien who takes the form of a witch and becomes intoxicated by human sensation, in the second-season Halloween episode Catspaw.
Antoinette Bower (born 1932) is a German-born British actress who worked widely in American television in the 1960s and 1970s.
In Catspaw she is Sylvia, one of two beings from another galaxy who have set up a Gothic castle on Pyris VII, drawing on what they take to be human fears. With her companion Korob she holds Kirk's landing party captive and controls Sulu, Scott and McCoy like puppets.
Unlike Korob, Sylvia is fascinated by the feelings her borrowed body gives her and grows cruel and ambitious. She tries to seduce Kirk, appears as a giant black cat, and crushes Korob beneath a door. When Kirk smashes the transmuter that sustains her illusions she is revealed as a tiny, fragile creature and dies.