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Sylvia was an alien from another galaxy. She and her companion Korob came in search of humanity, using their mental powers and a device called a transmuter. They established an outpost on the planet Pyris VII, fashioning a castle for themselves and human-like bodies. After luring the Enterprise to the planet, they captured the initial landing party comprising Scotty, Sulu and Jackson. Sylvia took control of their minds and sent Jackson back to the Enterprise where she killed him with a thought.
When Kirk, Spock and McCoy followed them down, Sylvia first appeared to them as a cat, then adopted human form. She demonstrated her power by creating a model of the Enterprise and first holding it to a candle and then surrounding it with a forcefield. She took control of McCoy's mind and, revelling in the sensation of her new body, tried to seduce Kirk. She enjoyed the pleasure of inflicting pain and took the form of a giant cat to menace Kirk and Spock, apparently killing Korob.
Kirk finally confronted her and seized the transmuter, smashing it. The castle vanished and the hypnotised men were freed. Sylvia and Korob reverted to their true form of small creatures who quickly died.