10. The Silence
First encountered in the episodes “The Impossible Astronaut” / “Day of the Moon” (2011), the Silence are a religious order who are intent on killing the Doctor to prevent him answering the oldest question in the universe, Doctor Who? They believe silence must fall when this question is asked and have made a number attempts to destroy the Doctor, including rigging his TARDIS to explode. Perhaps the most notable aspect of this particular monster is that they have been living alongside the human race for thousands of years and have the chilling ability of only being remembered when they are seen. This means as soon as you look away or turn your back, all knowledge of their existence is forgotten. On the request of President Nixon, the Doctor managed to defeat a group of them by splicing footage of a Silent saying “You should kill us all on sight” in amongst the broadcast of the first moon landing. This means the human race are driven to destroy any Silence they encounter, without even knowing it.