The Kovarian Chapter was a faction of the Church of Silence led by Madame Kovarian, which included several Silents. Circa the 52nd century, (TV: A Good Man Goes to War) after the Siege of Trenzalore broke out and the Church was assisting the Eleventh Doctor with protecting the planet from various hostile forces, preventing him from releasing the Time Lords back into the universe and restarting the Time War, (TV: The Time of the Doctor) the Kovarian Chapter decided to travel back in time and kill the Doctor, thereby insuring that the Time Lords never returned.
They made two (seemingly separate) attempts to kill the Doctor, the first by taking control of his TARDIS and causing it to explode on 26 June 2010. This, however, resulted in a total event collapse that, although corrected, left the cracks in the skin of the universe. (TV: The Pandorica Opens)
The second attempt involved the Silents influencing human history for thousands of years, culminating in the creation of an astronaut suit that would be operated by River Song, whom they kidnapped and raised to kill the Doctor (TV: A Good Man Goes to War) at Lake Silencio, Utah. Because it was a still point in time, they hoped to create a fixed point for the Doctor's death. (TV: The Impossible Astronaut/Day of the Moon) However, River refused to kill the Doctor, resulting in all of time happening at once, before this was corrected by the Doctor faking his death with a Teselecta. (TV: The Wedding of River Song)
The Doctor later commented that the Kovarian Chapter was caught in a "destiny trap," saying that you couldn't change history if you were part of it, since destroying the TARDIS created the crack through which the Time Lords could return, which they wanted to prevent. (TV: The Time of the Doctor)