Shapeshifting rock creatures of Excalbia who staged a battle between recreated heroes and villains to study good and evil.
The Excalbians are silicon-based beings living on the molten planet Excalbia. They can assume any form and conjure convincing replicas of figures from the past, which they use to study concepts foreign to them. Their spokesman, Yarnek, appeared as a living rock formation.
To explore the human ideas of good and evil, the Excalbians recreated Abraham Lincoln and the Vulcan philosopher Surak and set them beside Kirk and Spock against a team of history's villains, including Genghis Khan, Colonel Green and the Klingon Kahless. Kirk and Spock prevailed, and Yarnek concluded that good and evil used similar methods.