A gloomy, fog-shrouded planet where two alien explorers stage a haunted-castle illusion to test the Enterprise crew.
Pyris VII is a planet where an Enterprise landing party disappears at the start of Catspaw. Kirk, Spock and McCoy find a foggy landscape, a trio of witches warning them away and a forbidding castle, all illusions created by the aliens Korob and Sylvia.
The two explorers use a device called the transmuter to shape matter and minds, and they hold the crew captive inside the castle. When Kirk destroys the device, the illusions vanish and the aliens' true, tiny forms are left exposed and dying.