Silence
The Silence were first mentioned during Series 5 of the revived Doctor Who by Prisoner Zero and again by Rosanna but were not seen until the first two episodes of Series 6. The Silence appear as tall humanoids with bulbous heads, a mouthless bony face with eyes sunken in their sockets and the skin of their cheeks stretches to the points of their narrow cheek bones. They have large hands with 3 fingers and speak in a low, guttural voice. Though they have no mouths, a hole where you would expect a mouth to appear while absorbing and discharging energy when killing someone.
An individual member of the race is called a Silent, and were seen wearing black business suits during their on screen appearance in Series 6. The Silence are difficult aliens to beat, as you forget you have ever seen them the moment you turn your eyes off them. This gives them the advantage of sitting in the subconscious to play with humans, and is thought to have inspired Edvard Munch’s ‘The Scream’ painting. It is also believed that the Silence got humans to travel to the Moon in 1969 as they required a spacesuit being developed. Although the Silence were able to wipe themselves from memory, they were forced to abandon Earth (where they had lived for many centuries) when the Doctor, with the help of Canton, sent a broadcast from a Silent saying that humans should “kill us all on sight” which results in the humans turning on members of the Silence every time they see them.
The Silence were also responsible for the TARDIS exploding on June 26th, 2010 and thus creating the cracks in time that erased things from the universe, including the universe itself at the end.