Tarlak was a Voord Overlord and a key figure in the Voord invasion of Hydra. He described Yartek as his brother. He was intrigued into how Susan Foreman knew of Yartek and his mission to Marinus. He also explained how people became Voord when converting Pan Vexel. He tried to stop the First Doctor defeating the invasion by trying to convert Susan into a Voord. (AUDIO: Domain of the Voord)