The Thasians are a psionically powerful, non-corporeal species, native to the planet Thasus. Until the 23rd century, the Federation had never encountered the Thasians and thus believed them to be a myth.
However, a transport vessel crashlanded on Thasus in 2252 and only a small child, named Charles Evans, survived the crash. Instead of allowing the child to die, the Thasians granted Charlie with some of their psionic powers so that he could take care of his own needs on the barren planet. In 2266, Charlie was rescued by the crew of the Starfleet surveyor USS Antares, and from there taken to the USS Enterprise, where he would be transported to the Alpha V colony and live with surviving relatives.
While aboard the Enterprise, Charlie began to use his powers to influence the crew and their actions. Before he could create even more chaos on Alpha V, the Thasians managed to intercept the Enterprise and undid all of the damage that Charlie had done. When Captain James T. Kirk asked if Charlie could ever live among Humans again, the Thasians declared that he would always be tempted to use his powers, and transported Charlie back to Thasus. (TOS episode: "Charlie X")