Small, fragile creatures from another galaxy who used a device called a transmuter to appear as the wizard Korob and the witch Sylvia on Pyris VII.
The ornithoid lifeforms are visitors from outside the galaxy, sent ahead as scouts for those they call the Old Ones. In their true shape they are tiny, delicate things of down and thin limbs, barely able to survive unprotected.
They appear in the episode Catspaw. Using a transmuter that turns thought into matter, two of them take human form as Korob and Sylvia and build a castle of witches, dungeons and black cats from the fears they find in human minds, intending to test the Enterprise crew. Sylvia becomes intoxicated by physical sensation and grows cruel, while Korob tries to restrain her.
When Kirk smashes the transmuter the illusion vanishes. The two creatures are left on the bare rock and die within moments.