Decayed Master's possession of Tremas

"So, a new body at last."

- Decayed Master's last words

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)

History
The Decayed Master embarked on a scheme to steal the Source of Traken to extend his lifespan by becoming the Keeper of Traken. When he captured the Fourth Doctor within his TARDIS by keying it to his bio-rhythms, the Master showed interest in taking his body, but was thwarted when he was removed from the Source. The Master managed to escape in his other TARDIS, and it's incongruous form of a grandfather clock lured the curious Tremas to him, whereupon he was frozen in place with the same method as the Doctor was. Emerging from his ship, the Master (TV: The Keeper of Traken), using what energy he had managed to absorb from the Source (TV: Logopolis), gleefully merged with ,taking full possession of his body and killing his host in the process. Now in a rejuvenated body, the new rendition of the Master left Traken just as Nyssa began looking for her father (TV: The Keeper of Traken).

Legacy
While the Master would say that his new form was "not exactly" a regeneration in the typical sense (TV: The Five Doctors), the Fifth Doctor considered it one (AUDIO: The Light at the End), particularly as it has the same effect a typical regeneration would have. The Fourth Doctor was able to work out what had happened after he received a message from Nyssa telling of her father's disappearance. The Master used his new form to fool Nyssa for a time, until she discovered the truth. (TV: Logopolis) The Eleven's past incarnations would remember the Traken Source as one method the Master used to extend his life. (AUDIO: Planet of Dust)

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.