What was that alien in Doctor Who that nobody saw?

Question

There was this episode in Doctor Who where there is this alien that adapted itself so nobody could see it.

I don't know if it was in that episode where he goes to the end of the universe to the last planet and somebody knocks on the door from the outside. The Doctor then says that's impossible because there should not be anyone alive in the universe.

It could also have been the episode where he looks around in his Tardis and there are boards. He writes on them and when he looks again someone has written something under it.

I don't know when both episodes happened or with which Doctor so I can't check, but I hope somebody else remembers.

PS: if you know the two episodes I would also like to know which they where because I want to see them again.

Answer 1

This is "Listen"

This is a single episode, "Listen" (Season 8, Episode 4). The alien does not have a name, but it is a hypothetical "perfect hider."

From the description on the Doctor Who wiki:

The Twelfth Doctor is meditating on top of his TARDIS as it hovers above Earth when suddenly he cries out “Listen.” Talking to himself, he postulates why people talk out loud when they are alone. He hypothesises that it is because people inherently know that they are not alone. He uses his chalkboard to mark down that evolution produces survival skills. He notes that there are perfect hunters and perfect defences, but wonders why there is no such creature that is perfect at hiding.

The Doctor speculates about the possibility of a creature that has evolved to become so good at hiding that no one can perceive it. From the transcript:

DOCTOR: Evolution perfects survival skills. There are perfect hunters. (He watches a lioness bring down a wildebeest.)

DOCTOR: There is perfect defence.

(A shoal of dazzling tuna, and a puffer fish inflates itself so the nasty spines stick out.)

DOCTOR: Question. Why is there no such thing as perfect hiding? Answer. How would you know? Logically, if evolution were to perfect a creature whose primary skill were to hide from view, how could you know it existed?

And yes, the creature then does draw on his chalkboard:

The Doctor goes to the end of the universe to try to find these creatures. And yes, they do knock:

DOCTOR: Suppose that there are creatures that live to hide. That only show themselves to the very young or the very old, or the mad, or anyone who wouldn’t be believed.

CLARA: Okay, so.

DOCTOR: What would those creatures do when everyone was gone? When there was only one man left standing in the universe?

(Bang, bang, bang.)

CLARA: What’s that?

DOCTOR: Potentially, the hull cooling.

CLARA: Potentially?

DOCTOR: Believably.

(Bang, bang, bang.)

DOCTOR: Someone knocking.