Sylvia was the junior member of a team from beyond the Milky Way Galaxy of two Ornithoid explorers in Human form who established an outpost, disguised as a large castle, on the planet Pyris VII. Landing parties from the USS Enterprise were trapped by Sylvia and her colleague Korob for observation in 2267.
In the place where these aliens came from, there was evidently no luxury and little sensation. Sylvia's sudden immersion in sensation and luxury caused her to become irrational; she discovered, too, that she liked inflicting pain and controlling others. Korob considered Sylvia a traitor and her actions a betrayal of the purpose for which the Old Ones dispatched the explorers. But by that time, Sylvia was past caring; in fact, she planned to dispose of Korob and seize his greater power for her own use.
Sylvia could alter her form at will, and could telepathically examine the minds of individuals near her. She could also control the minds of individuals telepathically; all of these feats were performed by her own mind, amplified by the power of a mechanism entrusted to her; in her Human and cat forms, this instrument appeared as a crystal pendant. In one of the landing party expeditions to Pyris VII, Sylvia projected an image of Crewman Jackson in her mind. When she killed the image, and knew it was dead, she murdered him for real.
Korob, recognizing that his entire mission was compromised, freed the Enterprise crewmen, and began to guide them out of the castle. But Sylvia interrupted this, confronting them in the form of an enormous cat. While attempting to escape her clutches, Korob was crushed and seriously injured by a large door that Sylvia knocked off its hinges; Kirk seized Korob's wand, the transmuter, and escaped through the ceiling. There, Sylvia sent her enthralled minions, McCoy, Scott, and Sulu, against Kirk and Spock; they defeated these men, only to be faced with Sylvia, still a gigantic cat. But by this point, Kirk understood the importance of the transmuter: even though he couldn't control it, he threatened Sylvia with it. Sylvia evidently could operate it only when in physical contact with it; despite the fact that Kirk was quite close to her, holding the device, she continued to demand that he give it to her. Finally, she tried to use a phaser to force him to give her the device; he responded by smashing it, which undid everything it had created, leaving only the frail forms of Sylvia and Korob, tiny creatures only a few centimeters long, and frail as feathers on the wind, quickly withering and dying. (TOS: "Catspaw")