Prisoner Zero

"Silence Doctor. Silence will fall."

- Prisoner Zero's last words to the Doctor

Life Cycle
Not much is known about 'Prisoner Zero's' life cycle but The Doctor states that a multiform can live for millenia (many thousands of years) and that 12 years is just a 'pit stop'.

Forms
Prisoner Zero was a shapeshifter, whose default form is that of a large worm with a Angler fish head but able to change from one form to another by making a psychic connection with another lifeform and using their memories. Through this process, Prisoner Zero could even resemble multiple lifeforms, including their voices. The other forms that it assumed include: Prisoner Zero could also duplicate the voices of those whose image it took. However, in the case of multiple lifeforms, he had trouble speaking with the right voice through the right mouth.
 * Barney Collins and Dog
 * Lady with two girls
 * Amelia Pond and the Doctor

History
Prisoner Zero was a captive of an Atraxi extra-dimensional prison who escaped through a crack in the universe to Amelia Pond's house on Earth. Prisoner Zero hid within a room in Amelia's house, placing a Perception filter on the door to the room so that it would not be found. There it hid for twelve years, sometimes venturing outside by taking the form of humans with whom it had made a psychic connection and then rendered comatose.

The Doctor belatedly returned to Amelia's house - now known as Amy - followed by the Atraxi who broadcast a global warning that they would incinerate Earth if Prisoner Zero did not surrender. Prisoner Zero fled. The Doctor and Amy tracked it to the local hospital. There the creature revealed a ominous prescience, identifying the Doctor as a Time Lord and taunting him that "The universe is cracked. The Pandorica will open. Silence will fall". Believing that surrender would mean execution, Prisoner Zero said it was prepared to die in flames with the whole planet. When cornered by the Doctor, it tried to avoid Atraxi detection by making a psychic connection with Amy and using her memories to imitate her younger self and the Doctor. The Doctor thwarted this by stimulating Amy's recent memory of Prisoner Zero and forcing it to take on its own form. The Atraxi detected and secured Prisoner Zero, as it gave its final warning that "Silence, Doctor. Silence will fall" (DW: The Eleventh Hour).

Trivia
Prisoner Zero strongly resembles Razortooth from the SyFy original movie of the same name.