Spoiler alert, assuming you believe rumors!
From Doctor Who TV:
The Master has been rumoured to return in Peter Capaldi’s debut series for a while now, and now a certain actor very familiar with the Whoniverse may have let it slip.
According to attendees at a recent Newcastle Film and Comic Con panel, the 7th Doctor himself Sylvester McCoy claimed the Master is indeed back, but it won’t be John Simm in the role. McCoy didn’t reveal who exactly had been cast, but apparently said the actor in question is “very scary”.
With only tweets to go on, you’ll want to take this with a pinch of salt. You also have to question why McCoy, someone not even involved in the current production, would be so careless as to reveal it publicly given the usual BBC security around spoilers. It’s equally possible McCoy could just be reading into previous rumours or joking around.
Assuming there’s more to it, the name coming up frequently has been veteran actor Charles Dance. As usual whether there’s anything in this remains to be seen…