Gorgan (also known as the "Friendly Angel") was a noncorporeal being preserved in a cave on the planet Triacus. Spock's research suggested that Gorgan was possibly the last survivor of a race of marauders who used Triacus as a base from which to marauding the Epsilon Indi star system, until their victims rose up and destroyed them. The same space legends suggested that one survivor fled to a cave where it awaited the right "catalyst" to send it once again marauding across the galaxy.
In 2268, the Starnes Exploration Party accidentally released Gorgan from his cave. He used the party's children to evoke the fears of the adults, eventually rendering the adults mentally unstable and driving them to commit mass suicide. He further manipulated the children to follow him in his quest of conquering the universe.
On stardate 5029, Gorgan used the children to establish mind control over the crew of the USS Enterprise and forced the starship on a heading for Marcos XII. Gorgan hoped that nearly a million children on Marcos would join the other children as their friends. The rest would be their enemies, to be destroyed.
The children could collectively summon Gorgan by twice chanting:
(The children chanted "High, high fire and snow", instead of "Hail, hail", during their second summoning of the Gorgan.)
Gorgan was defeated by Captain Kirk, when he turned the children on him by recalling their lives with, and deaths of, their parents. Losing the children's obedience, Gorgan lost his power and dissolved into oblivion, but not before revealing his more monstrous-looking form. (TOS: "And the Children Shall Lead")