Prisoner Zero

Prisoner Zero was a multi-form shape-shifting alien, held prisoner by the Atraxi for unknown crimes. He escaped to Earth when the TARDIS explosion caused cracks throughout time and space, hiding in an extra room of Amelia Pond's home for twelve years until he was tricked into revealing himself by the freshly regenerated Eleventh Doctor and recaptured by his jailers to be executed. Before being taken away, he threatened the Doctor with the Silence.