Obviously, spoilers follow for the season 10 finale. (Even the question title is slightly spoiler-y, but it can't be helped.) OK, so finds Bill and turns her into, well, whatever it is that Indeed, even as the Cyberman-shell is still in the process of toppling, we see that Bill is Th...

[Yeah, if you haven't seen the season 10 finale, don't go reading this question, right?] This is probably supposed to be obvious, but who is that at the very end of the The Doctor Falls? Both in-universe and out, if you could, because part of my confusion is the out-of-universe fact that, as far...