Theros was the home world of the Therovians.
Grig left the planet to find a cure for the disease afflicting his people, later returning with the First Doctor, John and Gillian. They learned from Wopan that a cure for the plague had been found on Ixon. After travelling there and collecting it, they returned to Theros and saved the Therovians. (COMIC: The Therovian Quest)