Sarn was a volcanic planet which had been a colony of Trion. The Trions mainly used Sarn for its regenerating Numismaton Gas, but they began using it as a prison planet for political prisoners.
The people of Sarn descended from a high-technology society into tribalism and superstition. They worshiped Logar, God of the Fire Mountain, and sacrificed unbelievers in fire.
When Turlough and the Fifth Doctor went to Sarn, Turlough sent a distress call to Trion, requesting assistance for the people, who were threatened by a volcanic eruption during the Time of Fire. (Planet of Fire)
Behind the scenes
The Sarn landscape was depicted through filming in Lanzarote in the Canary Islands.