10 Best One-Time Only Doctor Who Villains
3. The Veil (Heaven Sent)
While villains that have to be dealt with head-on are definitely intimidating, there's something much scarier about a monster that - even though it might not be an immediate threat - will never, under any circumstances, stop chasing you.
And that's exactly what The Veil is all about. It's a slow, unarmed, easy-to-spot creature, which doesn't sound too scary when you put it like that. But here's something that is scary: it will never stop hunting you.
You could lock yourself in a room for months on end, but when you open the door again, the Veil will still be there, patient, always waiting. The Twelfth Doctor encountered this creature while trapped inside a labyrinthine castle, and ended up staying there for billions of years, the Veil breathing down his neck the entire time.
It's a horrifying thought that takes "you can run, but you can't hide" to the absolute extreme, and the Doctor has never had to work harder to outplay a villain, just one of the reasons why the Veil is among modern Who's most memorable enemies.