Excalbia was an inhabited planet. This was the homeworld for the Excalbians, a silicon-based intelligent species.
The planet had a poisonous atmosphere and a surface that consisted mostly of molten lava. The planet was located hundreds of light years from Earth.
In 2269, the USS Enterprise was sent to Excalbia by Starfleet to confirm several "old space legends" about the planet, which suggested that the planet was home to intelligent life. Orbital scans of Excalbia revealed several scientifically anomalous readings, including evidence of carbon cycle lifeforms on the surface, as well as indications of artificial power being generated in factor seven quantities, indicating the presence of a considerable civilization.
As the Enterprise prepared to warp out of orbit, the ship was subjected to an incredibly swift deep probe from the planet. Moments later the ship was contacted by the image of Abraham Lincoln, who eventually convinced Captain Kirk and Spock down to Excalbia's surface, where they were subjected to an Excalbian experiment on the nature of good and evil conducted by Yarnek with Kirk and Spock's unwilling participation. Yarnek recreated several historical figures thought to embody the concepts of good and evil and pitted them in combat against each other before the Enterprise crew was allowed to depart. (TOS: "The Savage Curtain")