Geoffrey Beevers was a guy they dragged in to briefly play the Master as a placeholder until they could get a Roger Delgado lookalike in the role. Peter Pratt was too busy doing opera or whatever, so they had no choice but to recast. The BBC weren't satisfied because the change in actor wasn't obvious enough, so they removed the golfball eyes and skeletal jaw just to really rub it in people's faces.
Originally only intended to appear in part 4 of The Keypuhh of Trahkahn and then be forgotten about. However, when Briggs was trying to get actors onboard for his super duper canon adventures, he discovered that every other classic Master actor was a little bit too dead to participate. Not resisting an opportunity to milk a character of any and all storytelling potential as possible, Briggs hired Beavers to resume his incarnation of the Master. Beavers went on to be in many fan favourite audios, including Mattress and The Five Doctors Part 2 Erm, 3.
"A new body... at last!" - Kinky.
"You're going to time ram me?!" - The Master does not enjoy anal.
"JUST SHUT UP AND FUCK ME, JACQUELINE!" - The plot of Master.