Excalbian

The Excalbians are a shapeshifting silicon-based lifeform, native to the planet Excalbia, described as being "almost mineral" and "like living rock with heavy fore-claws."

In 2269, the Excalbians became intrigued by Human notions of good and evil.

They staged a contest between the two ideas of thought, using Captain Kirk and Commander Spock of the USS Enterprise, and recreations of notable galactic historical figures.

Representing "good", were Kirk, Spock, Abraham Lincoln of Earth and Surak of Vulcan; and representing "evil" were Kahless the Unforgettable of Qo'noS, Genghis Khan and Colonel Green of Earth and Dr. Zora of Tiburon. (TOS: "The Savage Curtain")

Information about Excalbians was displayed on a computer graphic in Keiko O'Brien's classroom on Deep Space 9 in 2269 as a part of her lesson on comparative xenobiology. (DS9: "A Man Alone", "The Nagus")