The Sisters of Silence were an Imperial organization created by Malcador to assist in dealing with rogue psykers. The Sisters were an all-female force of psychic blanks, and took vows of silence. They were present in large numbers during the Siege of Prospero, and the War in the Webway.
After the Horus Heresy, the Sisters disbanded as an official organization, but continued to recruit members and serve the Emperor's ideals. They have since fought during the War of the Beast and the Gathering Storm.