The Excalbians were a non-humanoid carbon cycle life form native to the planet Excalbia. Prior to the discovery of this species, the notion of intelligent life on Excalbia was considered little more than "old space legends".
In 2269, the USS Enterprise was sent by Starfleet to investigate Excalbia, but found little outside a poisonous atmosphere and a molten lava surface. Sensor readings of Excalbia indicated the presence of artificial power generation on the planet that registered "in factor seven quantities," indicating the presence of "a considerable civilization there."
Though the condition of their planet should have deemed the planet's inhabitants' existence as scientifically impossible, they nonetheless registered as carbon-cycle lifeforms. On the Enterprise's final orbit, the Excalbians scanned the ship, including the minds of the crew, via deep probe scanning technology, noted as being "incredibly swift." Moments thereafter, they initiated first contact through a re-creation of Abraham Lincoln, one of James T. Kirk's personal heroes.
Prior to beaming "Lincoln" up to the Enterprise, sensors described the being "almost mineral" and "like living rock with heavy fore-claws", before finally registering as Human. Spock later observed the Excalbians appeared to possess the "ability to control matter and to rearrange molecules in whatever fashion was desired." This allowed them to create other beings, who appeared or registered as humanoid, by "using their fellow creatures as source matter." These forms taken by Excalbians truly believed and behaved like the person they portrayed.
In their natural form, Excalbians appeared as a simple rock formation that possessed glowing eyes, a distinguishable head and two arm-list appendages. Also in their natural form, Excalbians radiated extreme body heat.
During the interactions with the crew of the Enterprise, through Lincoln, the Excalbians became intrigued by the Human notions of good and evil. They staged a drama, or play, between the two concepts, using Kirk and Spock and recreations of notable galactic historical figures, enacted on an Earth-like stage on Excalbia's surface.
Then it was announced that countless spectators were there because Excalbians learned by "observing such spectacles," by offering Kirk and Spock the honor and opportunity "to become our teachers by demonstrating whether good or evil is more powerful. Representing "good" were Kirk, Spock, and recreations of Abraham Lincoln of Earth and Surak of Vulcan; representing "evil" were recreations of Kahless the Unforgettable of Qo'noS, Genghis Khan and Colonel Green of Earth, and Zora of Tiburon."
When it was determined by the Excalbians that the stakes were not high enough to evoke the response they sought, they demonstrated their ability to disable the Enterprise, by slowly forcing it to operate on battery power alone, before causing the breakdown in the shielding between the ship's matter and antimatter.
Though "good" was victorious in the end, the Excalbian Yarnek felt that Kirk and Spock "failed to demonstrate to me any other difference between your philosophies." When Kirk asked them, "[w]hat gives you the right to hand out life and death?" Yarnek replied, "The same right that brought you here. The need to know new things." (TOS: "The Savage Curtain")
Information about Excalbians was displayed on a computer graphic in Keiko O'Brien's classroom on Deep Space 9 in 2369, as a part of her lesson on "Comparative Xenobiology". (DS9: "A Man Alone", "The Nagus" okudagram)
The Excalbians were not identified by name on screen or in the script. The name is generally sourced to several reference works, including the Star Trek Encyclopedia, Star Trek Chronology, Star Trek: Star Charts, and StarTrek.com. On the Comparative Xenobiology chart, they were referred to simply as "carbon cycle lifeform from Excalbia". The name, however, does appear in an okudagram which references the "Excalbian Campaign".
According to the first draft of the script, this species was described as insectoid with a "hard-shell body, heavy fore-claws."
The Pocket TOS novel Savage Trade has the Enterprise encountering a group of renegade Excalbians who wish to apply for refugee status to the Federation. They were forced to take permanent human form, again notable members of Earth's history.