Thasus (also known as Thasus IV) was a inhospitable planet located somewhere in the space of the galaxy's Alpha or Beta Quadrants, the fourth planet in orbit of the Thasus star system, at coordinates 4.97S 2.23W. Thasus was the homeworld of a non-corporeal species known as the Thasians. (FASA RPG module: The Federation)
Prior to the 2250s, Federation surveys of the planet reported some highly sophisticated artifacts located on the planet's surface, but indicated that conditions hadn't changed on the planet for at least three million years. The survey also indicated that the Thasians had once thrived on the planet millennia ago. (TOS novelization: Charlie X)
In 2252, a transport carrying a shipload of passengers crashlanded on the surface of the planet. The only survivor of the crash was Charles Evans, a three-year-old human child. With the child facing certain death, the Thasians took pity upon him and granted him some of their psionic powers, which allow him to care for himself by creating anything that he needed out of thin air.
In 2266, Charlie (now 17) was able to escape from the planet aboard the Federation cargo vessel USS Antares and used his psionic powers to control its crew. Eventually, Charlie was transferred to the USS Enterprise, which he also tried to take control. Before he could do much damage, the Thasians came to take Charlie back to Thasus. (TOS episode & novelization: Charlie X)