Doctor Who: 10 Villains The Doctor Couldn't Defeat

2. The Black Guardian (Enlightenment)

The Black Guardian hasn't been seen in a TV episode since 1983's Fifth Doctor serial Enlightenment, and although he burst into flames at the end of that adventure, he hasn't really been defeated: to quote the White Guardian "he will be waiting".

This is because the Black Guardian is pretty much a god. A personification of the forces of darkness, the Black Guardian is only balanced out against the White Guardian, who personifies the forces of light: while one exists, the other exists too.

And so, you can get rid of the Black Guardian for a while - which is what happened after that aforementioned flames incident, which was caused by Turlough throwing a diamond at him - but it's not like you can just kill him for good.

As a result, this is an enemy who could easily return at some point in the future, and that would definitely be an interesting day for the Doctor and her companions.