"Vaal is Vaal. He is – everything."
By 2267, when the planet was visited by Captain James T. Kirk and the Enterprise, the planet was inhabited by a primitive culture of humanoids called Vaalians, who were slaves to Vaal and revered it as a god.
Vaal was ultimately destroyed by Kirk and the crew of the Enterprise when it killed several crew members and was on the verge of destroying the Enterprise and all her crew by pulling them into the planet's atmosphere. This action by Kirk may have been in direct violation of the Prime Directive, but Kirk felt justified that the people on the planet needed to be freed from their false god in order to achieve their full potential. (TOS: "The Apple")