The Decayed Master, in search of a body
Peter Pratt
The Master returned for the first time in Season 14, where he was played by Peter Pratt. Having used up all of his regenerations, the Master was looking for a way to extend his life. He decided to do so with the use of Gallifreyan technology, framing the Doctor for murder along the way. While the Doctor defeated him, he was able to claim a TARDIS and escape.
Peter Pratt's version of the Master appeared in all four episodes of "The Deadly Assassin" serial, episodes 9 through 12 of Season 14.
Geoffrey Beevers
While he was partially healed by his actions in "The Deadly Assassin," the Master was still dying and in need of a new body. Rather than having Peter Pratt return, they recast the Master with Geoffrey Beevers, who is nonetheless intended to be the same version of the Master. His next attempt at finding a new body came in "The Keeper of Traken" serial in Season 18, Episodes 21-24.
He succeeded in his goal in the final episode of the serial, allowing the next actor to take on the iconic role.