However, Kassia came under the influence of the Melkur and she turned against the Traken Union. When Tremas was considered unfit to become Keeper for helping the Fourth Doctor and Adric, Kassia was chosen. When she became connected to the Source, the power of the Keepers, she was destroyed and replaced by the Melkur, who it was revealed was really the Master. Tremas helped the Doctor stop the Master and still save the Source.
Shortly after the Doctor and Adric left Traken, Tremas noticed a grandfather clock. As he went to investigate it, he became immobilised on touching it and the Master emerged. With some of the Keeper's powers still lingering, the Master assimilated Tremas's body. As it became younger in appearance, the Master gleefully chuckled over his new body, and left Traken. (TV: The Keeper of Traken)
Tremas's fate greatly troubled his daughter Nyssa. Due to the Trakenite belief in the connection between the soul and the body, she believed that Tremas could never be at peace while the Master was using his form. (PROSE: Empire of Death)