Catspaw

Crewman Jackson, part of a landing party on Pyris VII, beams back to the Enterprise and collapses dead, and an eerie voice warns the ship to leave. Kirk, Spock and McCoy go down to find the missing Scotty and Sulu. In the fog they meet three ghostly witches, and then a gloomy castle. Inside are Korob, a wizard-like figure, and Sylvia, a woman who can take the form of a black cat. Scotty and Sulu reappear, but under the aliens' control. The pair are visitors from another galaxy who use a device called a transmuter to create illusions and reshape matter. Sylvia shows her power by holding a miniature Enterprise over a candle flame, heating the real ship's hull. Experiencing human sensations has made her cruel and hungry for power, and Korob turns against her, helping the prisoners escape. Kirk smashes the transmuter, and the aliens are revealed in their true forms as tiny creatures who cannot survive without it.