In 2267, the ornithoid lifeforms from outside the Milky Way Galaxy, Sylvia and Korob, established an outpost there for the purpose of learning about the worlds of the Federation. Sylvia became infatuated with sensation, and dangerously irrational. To stop her, James T. Kirk destroyed the transmuter used by the aliens. Without its power, the two aliens were not able to survive on the surface of Pyris VII and died. (TOS: "Catspaw")
The original appearance of Pyris VII was replaced when the episode "Catspaw" was remastered in 2006. Additionally, a new matte was created to represent the castle located on the planet, using the original shot of the fog-shrouded entryway and building the full castle on top of it.
According to Star Trek Maps (p. 29), Pyris was a blue dwarf star with ten planets.