Well that was a blast from the past...

Following a fakeout where it's theorized that the mysterious woman popping up in episode after episode may be the long lost Susan Foreman (last seen in the episode "The Five Doctors" in 1983), it turns out they were the villain Sutekh, last seen in the episode "Pyramids of Mars" in 1975.

https://en.m.wikipedia.org/wiki/Susan_Foreman

https://en.m.wikipedia.org/wiki/The_Five_Doctors

https://en.m.wikipedia.org/wiki/Pyramids_of_Mars

Season Finale next week!