Academy of the Questedition/The Sentinels of History
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The Silence was a religious order who considered themselves the “Sentinels of History”. They tried to kill The Doctor to prevent the fruition of a specific prophecy, which stated:”On the fields of Trenzalore, at the fall of the Eleventh, when no living creature can speak falsely or fail to answer, a Questedition will be asked, a question that must never, ever be answered.”
This question was “The First Questedition, the oldest question in the Universe, that must never be answered, hidden in plain sight.” Dorium Maldovar told the Eleventh Doctor that the question was:”Doctor Who?”, which was a question The Doctor had been apparently running from his entire life.
The Silence was led by a race of aliens who would be erased from your memory after you looked away from them (known as Silents) who used post-hypnotic suggestion to manipulate other species into doing their bidding. The Silents also employed human or humanoid agents to carry out specific tasks. These agents were often fitted with Eye Drives which allowed them to remember the Silents. (A Good Man Goes To War, The Wedding of River Song)
The Silence allied with Madame Kovarian, the Church and the Order of the Headless. They were behind the Battle of Demon’s Run and the kidnapping and brainwashing of the infant River Song, then known as Melody Pond. (A Good Man Goes To War) She later made several attempts on the The Doctor’s life. Ultimately, she kissed him with poisoned lipstick. She later broke free of her conditioning and offered him the total of her regenerative energy, saving him. (Let’s Kill Hitler)
The alien leaders of the Silence were encountered by The Doctor in 1969 America. The Silence had been on Earth since humanity’s beginning, secretly manipulating humanity and its development for their own ends.
It was implied they were responsible for the Moon landing because they needed a special spacesuit for Melody Pond. The Silence’s occupation of Earth was broken when The Doctor issued a post-hypnotic command via the Apollo 11 footage to the entire human race, instructing them to kill the Silents on sight. (Day of the Moon)
source TARDIS Wikia