The Decayed Master

This version of the Master appeared only a couple of times on TV, played by Peter Pratt in "The Deadly Assassin" and later by Geoffrey Beevers in "The Keeper of Traken." However, Geoffrey Beevers became a friend to Big Finish, being the husband of Caroline John and father of Daisy Ashford, both of whom have played Liz Shaw on audio. As the Master most associated with the Fourth Doctor, he has appeared in a number of Fourth Doctor Adventures stories, once Tom Baker returned to the role.

Through some strange logic, Big Finish has established that when the Master's stolen body wears out, he reverts to the Decayed incarnation. Why his personality and voice would change, I don't know, but there it is. As a result, there are some stories here set much later in the Master's timeline, but not very many.