The Melkot (or Melkotians) were a telepathic and xenophobic civilization. The Melkot used their telepathic powers to punish aliens who encroached upon their space without permission (which, prior to 2268, they never extended). A typical Melkotian physically floated without a body. It had a bulbous head and a pair of very bright, circular eyes.
The Federation made first contact with the Melkotians in 2268, when the USS Enterprise encountered an orbiting warning buoy. When Enterprise Captain James T. Kirk ignored the Melkot warning to leave their territory and instead assumed orbit and beamed down a landing party, the Melkotians passed harsh judgment on the landing party, describing them as "aliens" and "disease" and condemning them to death for their trespass. The form of execution was to be by telepathic projection of illusions that would be indistinguishable from reality and incorporate into them a believable lethality. This illusion took the form of the famous gunfight at the OK Corral in Tombstone, Arizona, on October 26, 1881, between the Earps and the Clantons, a fight that the Clantons lost. Kirk and the members of the Enterprise crew were to fill the part of the Clantons, but managed to avoid death by having their psyches hijacked by Spock, who used the mind meld to alter their perception of reality. Faced with the opportunity to kill the recreation of Wyatt Earp, Kirk instead discarded his weapon. At that point, the illusion dissolved and the Melkotian buoy exploded. The Melkot appeared on screen, intrigued by Kirk's display of mercy towards Earp, and suggested that the Enterprise visit Melkot, where a delegation would come out to meet them. (TOS: "Spectre of the Gun")
Even though the Melkotian was conceived as a form of floating and disembodied non-corporeal entity, it still had to be physically represented in a way more substantial than merely using some flickering lights. Edward K. Milkis therefore invited Mike Minor to design the alien being, after Minor managed to secure an active role in the production of Star Trek: The Original Series. "He called me back during Star Trek (the TV show)," recalled Minor, "and said, 'We need a critter. Would you be interested in doing it?' I said that I'd love to and it turned out [...] they needed the Melkotian [....] I came up with a miniature head which was a brain case, two eyes set deep into skull sockets, no nose and no mouth because I reasoned that they'd have evolved away from mandibles or having to eat, since there was no body. It was simply done for the effect of the moment." (Enterprise Incidents, issue #14, p. 44)
Although Mike Minor had accepted the task of representing the Melkotian without ever having done foam castings or latex pulls from molds, he firstly cast the alien in foam. (Starlog, issue 25, p. 36; Enterprise Incidents, issue #14, p. 44) After buying his first can of latex from Verne Langdon at Don Post Studios and receiving some instructions from him, Minor returned to working on the Melkotian. (Starlog, issue 25, p. 36) He recollected, "I sculpted it in clay, made a negative female plaster mold and developed the latex into a half-inch thick mass, which is flexible." (Enterprise Incidents, issue #14, p. 44)
The Melkotian underwent some last-minute changes before being filmed. "It was shot over at one of the optical houses which worked on the show (there were three or four)," Mike Minor remembered. "We shot it against black and put pingpong [sic] balls in the eyes, cut cat-like slit pupils and put high intensity tensor beams behind it, put a star filter over the lens. This was shot, and this image was literally double-exposed over a cloud effect on the stage. It made about three appearances in the show. So that was the first on-screen effect that I did anywhere." (Enterprise Incidents, issue #14, p. 44) Minor concluded, "That job turned out very well." (Starlog, issue 25, p. 36)
In the episode's novelization, the Melkotian is described as a tall, thin humanoid wearing a robe.