Korob was one of two Ornithoids who established an outpost (in the form of a large castle) on Pyris VII around 2267. Sent there from outside the galaxy by the Old Ones, their declared goal was to learn about Humankind.
To this end, Korob devised a series of frightening images intended to test the loyalty, bravery, and character of visitors. These images were drawn telepathically from the subconscious recesses of his visitors' minds. He also wanted to better understand the science of his visitors; his questions made Captain Kirk uneasy, as they seemed too much like the kind of information advance agents for an unfriendly power might want to learn. Kirk's fears were compounded by the behavior of Korob's colleague Sylvia, who tormented the visitors. The aliens could reshape matter, transport people and objects from place to place, and create other effects that, to the unsophisticated eye, resembled magic.
On learning that Sylvia intended to dispose of him, Korob finally realized she had become dangerously irrational. He freed the landing party from Sylvia's cell, but was forced to retreat from Sylvia, who had assumed the form of an enormous cat. Attempting to escape, Kirk, Spock, and Korob returned to the cell, where Korob was crushed and severely injured when Sylvia battered down to the door. Kirk was able to retrieve Korob's wand, which proved to be the chief source of the aliens' power; it was a mechanism called a transmuter. To prevent Sylvia from seizing it, Kirk smashed it. All the alien artifacts disappeared; all that remained were the tiny, helpless forms of the aliens themselves, as Sylvia described them "like feathers in the wind". Within seconds, they perished, and dissolved into smoke. (TOS: "Catspaw")