Chag Gara was a 23rd century Kyrosian man, a regional religious leader on the planet Kyros. Born around the year 2226, he was well known in the city of Andros as a rampant womanizer. He believed himself to be the sole messiah of the gods who lured followers in with promises of houris in the afterlife.
The library computers on board the Federation starship USS Enterprise described Chag Gara as a "mentally disturbed fanatic with low intelligence and an inability to order his thoughts."
In 2269, he and Spock were mentally linked using a telescan cephalic implant which enabled Gara to use Spock's intellect to further his ambitions. The crew of the Enterprise recovered Spock from Gara and used radical therapy to "cure" him.
The library computer listed Mohammed, Stur, and Nerid as analogous examples to Chag Gara. (TOS novel: Spock, Messiah!)