Silent

This memory-proof alien species was heavily associated with the religious order known as the Silence. They were tall humanoids with bulbous heads and grey, mouthless faces. Their hands had four large, elongated fingers with one bulbous central finger. They had small eyes in deep recesses in the skull. The eyes lacked sclera and were completely dark. They spoke in low, guttural voices and tended to make growling or clicking noises. They wore black suits and ties in the style of 20th century male government workers on Earth. (DW: The Impossible Astronaut/Day of the Moon) This species would also hang upside down from ceilings while they slept, much like Earth bats. (DW: Day of the Moon) The true name of this particular species has not been revealed.

Powers and abilities
This species were able to remain hidden on Earth for thousands of years, as they possessed the unique quality of being "memory-proof": anyone who saw them - even a photograph or recording of them - would instantly forget about them the moment they looked away. Anyone looking at the aliens would recover their memories of previous sightings, although not instantaneously.

The aliens could use an electrical discharge from their hands to kill and obliterate humanoid beings, leaving only a little ash. It appeared that they had to draw this energy from an outside source since when Joy was killed by one of the members of the species, the lights in the restroom flickered. When absorbing energy and discharging it, a hole appeared where a mouth would have been on a human's face. (DW: The Impossible Astronaut / Day of the Moon)

The aliens could influence the actions of others by issuing commands to any person who looked at them. Any order so given would remain in a person's mind even after they forgot the encounter with the alien, and take on the form of post-hypnotic suggestion. This would also happen if the suggestion was made by someone other than one of the aliens to a person looking at an individual of the species or an image of one. People could be driven insane by having too many orders implanted in this manner. (DW: Day of the Moon)

History
The aliens had been present on Earth for thousands of years, since the time the wheel and fire were developed, going unnoticed by the Human race. They used their ability of post-hypnotic suggestion to guide Humans into doing their bidding in such things as going to the Moon. (DW: The Impossible Astronaut / Day of the Moon)

A single member of the species spied on the Doctor, Amy, Rory, and River in Utah in 2011, briefly being noticed by Amy before Rory distracted her. Another individual visited the White House in 1969 and killed a woman named Joy in front of Amy Pond before delivering a post-hypnotic suggestion for her to "tell the Doctor what he must know, and what he must never know". (DW: The Impossible Astronaut)

At some point, a great number of the aliens had taken up residence at a disused orphanage where the little girl was held. They used post-hypnotic suggestions to control the man that had run it into thinking the orphanage was still in operation. They showed great interest in the welfare of the little girl, putting her in the life support "astronaut" suit to keep her healthy and safe - even equipping the suit with a built-in weapons system. The aliens also kidnapped Amy when she was investigating the orphanage with Canton. They took her to their underground lair and cryptically told her that she would help "bring the Silence."

The Doctor inflicted a crushing defeat on the aliens when he ensured that Humanity saw footage of a wounded individual member of the species they held captive, spliced into the footage of the Moon landing broadcast. In it, the alien said the words "You should kill us all on sight", having been meant at the time as a jab at apparent Human mercy. This footage was subsequently viewed by 500 million Humans live, and would be seen every time anyone viewed the Apollo 11 landing footage in the future, one of the most famous and most-viewed film sequences in the history of the planet. Without even realizing it, Humanity began to exterminate the aliens as commanded. Those individuals holding Amy captive tried to kill the Doctor for this. However, River killed all the aliens that were present. Without enough time to implant more post-hypnotic suggestions when Humans looked at them, the remainder of the species on Earth in 1969 seemed doomed, although at least one did survive to 2011. (DW: Day of the Moon)

As the aliens were the leaders of the movement known as the Silence, and in order to interact with them, many of their operatives wore "eye drives", devices worn over the eye that artificially recorded information and fed it directly into the brain, allowing those wearing the devices to remember the aliens after looking away. Two of these creatures helped Madame Kovarian kidnap River Song from the Luna University and put her in an astronaut suit as part of Kovarian's plan to kill the Doctor. When River asked Kovarian what they were, Kovarian merely replied, "your owners." (DW: Closing Time)

In the Area 52 Pyramid in an altered timeline, at least a hundred of these creatures were kept in specimen tanks filled with water. They broke out and killed the guards. Three of them attempted to kill Rory but were defeated by Amy Pond. (DW: The Wedding of River Song).

Behind the scenes

 * Steven Moffat has stated that he was inspired by Edvard Munch's The Scream when creating the Silence's appearance.