Silent

The Silents are a sapient humanoid species which has a strange amnestic effect on those who observe them. As soon as one catches the sight of a Silent, one immediately remembers any previous encounters had with them; but as soon as one looks away from them, one forgets all of it.

Members of this species also lead the religious cult known as the Silence, which conspires to prevent the answering of the First Question: one which concerns the Doctor and which according to their prophecy, must never be answered.

Biology
The Silents are slim, hairless humanoids with pale grey skin, enlarged heads, bony features on their faces, a small mouth visible only when opened, and are invariably seen wearing black suits. They are known to sleep hanging upside down from ceilings, much like bats or Tetraps. They are capable of manipulating electricity and generating energetic discharges powerful enough to disintegrate a Human target with ease. They are physically stronger than most humanoids, and somewhat resistant to gun injury, although they can be killed my multiple gunshots. Whatever causes the amnesia they trigger in those who observe them is a result of their physical appearance, as seen by the fact that if one looks at merely a photograph or hologram of a Silent, the effect is the same: they will forget as soon as they turn away.

History
The Silents have been hidden on Earth since the dawn of Human civilization. They are the keepers of a huge integrated system of underground tunnels which go through the entire subsurface of Earth, yet remained always undiscovered due to the fact that everyone who ever encountered the creatures has forgotten it as soon as they left. Despite this, the Silents are able to leave subconscious suggestions on those who encounter them, and thus subtly influence their behavior. For millennia they have taken advantage of this ability by controlling Human society, influencing them to create any technology the Silents might want. They are described as parasites, for they have no need to create their own technology, instead using what other species create from their suggestions.

In 1969 the Doctor was able to use the Silents' own powers against them and free the Human population from their control. A recorded video of a Silent admitting that the most proper action for Humans would be to kill them all on sight, was inserted in the middle of the moon landing transmission, being watched by half a billion people all over the globe. The transmission would be replayed for generations and each Human who watches it would - although never remembering the inserted video of the Silent - retain the information in their unconscious mind. From that day on the Silents would never be able to interact with Humans without being attacked, and were presumably forced to leave the planet en masse, although at least one was definitely present at Lake Silencio in 2011, 42 years after the moon landing.

Trivia

 * Their appearance is somewhat reminiscent of the fictional urban legend known as the Slender Man, especially in the limited facial features and the ever-present black suit.