Silents

"You should kill us all on sight."

- One of the Silents

The Silence was thought of as a malevolent alien race from Doctor Who revealed later to be part of a religious order with their nature revealed near the end of the Eleventh Doctor's life. In essence, The Silence embody the fear of the unknown (or more accurately the unknowable) as their ability to make those who seen them instantly forget make them invisible to all but the most clever of minds.

History
The Silence started out in Earth's distant future as Confessional Priests — genetically engineered by a military organization known as the Church of the Papal Mainframe. When the Papal Mainframe received a signal from Trenzalore, it created a barrier to keep other races like Daleks and Cybermen from entering though all races were nervous to venture to the source due to the dread it posed. When the Eleventh Doctor translates the signal, translated as the oldest question "Doctor Who?", a siege begins with the Papal Mainframe reaffirming their faith to "silence will fall" with the intent not to allow the Doctor to speak his name as it would bring back the Time Lords with a new Time War as a consequence.

While the main body of the church served to keep the Doctor's enemies at bay, a chapel of the chruch under Madame Kovarian decided to venture into the Eleventh Doctor's past. However, as seen when they first blow up the Doctor's TARDIS, the chapel caused the cracks in time and space from which the Time Lords' signal originates from. Furthermore, revealed to have influenced mankind's development in the process such as the moon land, the second attempt on the Doctor's life by the Kovarian Chapel's Silence using River Song only ensured that Doctor is able to reach Trenzalore. The Silence loyal to the Papal Mainframe remained behind and later aid the Doctor when the siege becomes a full blown war lasting centuries.

Trivia

 * The Silence bare a resemblance to the famous Buffy monsters, The Gentlemen - as well as familiar psychological tricks that have become a staple of modern Doctor Who.
 * Their Appearance is very similar to Slender Man. The both ware suits, have grey/white skin, and faces with little to no facial features.