Decayed Master's possession of Tremas

Using the Source of Traken, the Decayed Master possessed the body of Tremas, a Trakenite, (TV: The Keeper of Traken) becoming a new, distinct incarnation (AUDIO: Masterful) who would admit that the process was "not exactly" a regeneration, (TV: The Five Doctors) whilst Tegan Jovanka and Nyssa learnt from the Doctor that it was indeed the Master's thirteenth regeneration. (PROSE: Divided Loyalties) This possession resulted in the death of Tremas. (TV: Logopolis)

Behind the scenes

 * The original second volume of The Doctor Who Programme Guide (1981) stated that the Master had used Tremas's body for his "thirteenth regeneration", implicitly designating the resulting "Tremas Master" as the Master's fourteenth incarnation. This would be contradicted in The Five Doctors, in which the Master admits to the Third Doctor that his new appearance is "not exactly" a regeneration.