A molten planet of living rock beings who stage a battle between good and evil in The Savage Curtain.
Excalbia is a planet with a molten surface and a toxic atmosphere, home to the Excalbians, rock-like beings with the power to create matter and life. They had never encountered the human concepts of good and evil.
In The Savage Curtain, the Excalbians create a habitable area on Excalbia for the contest between Kirk's team and a team of villains, while holding the Enterprise in orbit. The experiment ends when the villains flee and the Enterprise is released.