THE SILENCE

The Silence are humanoids ressembling grey aliens. They stand nearly 2 meters tall with bulbous heads and grey, mouthless faces. Their hands possessed large, elongated fingers with a bulbous central finger. They speak in low, guttural voices and tended to make growling or clicking noises. Their bodies are covered with what appears to be black suites. As soon as someone is not looking at a Silence, its memories of it blanks out of his memory but remains in his unconscious as upon looking on a Slience again, previous memories of meetings come bac (also to blank out as soon as visual contact is lost). They absorb energy from nearby energy sources and discharge it from their hands with enough power to disintegrate a human. When absorbing the energy and discharging it, a hole appeared where a mouth should have been. They can influence humans by saying something while a person was looking at them, which would remain in there heads after they looked away in a form of post-hypnotic surgesion. Constantly doing this would cause the person in question to lose their mind.
The Silence were apparently present on Earth for hundreds of years since the time the wheel and fire were developed, going unnoticed by the human race. The Silence were defeated by the Doctor when he ensured humanity saw footage of a Silent ordering people to kill them on sight, broadcasted via the moon landing. This 'command' acted like a post-hypnotic suggestion and resulted in the human race finally turning on the Silence.
Source:Doctor WHo Universe : The Impossible Astronaut, Day of the Moon