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, able to change from one form to another by forming a psychic connection with another lifeform, and using an image from within their mind. Through this process, Prisoner Zero could even resemble multiple lifeforms, which included their voices. The forms that it took on included; 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
As his given title would suggest, Prisoner Zero was a captive of the Atraxi, imprisoned within an extra-dimensional prison. At one point, he was able to escape through a crack in the universe, arriving in Amelia Pond's house on Earth. Prisoner Zero hid itself within one the rooms of Amelia's house, placing a Perception filter on the door so she would not notice it. He remained in hiding for twelve years, occasionally venturing outside by taking the form of those he made a psychic connection with, rendering them comatose as a result.

When the Doctor returned to Amelia (now calling herself Amy)'s house, he brought the Atraxi with him, who threatened to incinerate the whole planet if Prisoner Zero did not reveal himself. Realising that turning himself in would mean execution, Prisoner Zero ran, preferring that the entire planet die with him. The Doctor and Amy tracked him down to the local hospital. There it was revealed that the creature was unusually knowledged about the universe; identifying the Doctor as a timelord and aware of events occuring in time that the Doctor wasnt. Prisoner Zero taunted the Doctor, telling him "The universe is cracked. The Pandorica will open. Silence will fall". In a last-ditch attempt to avoid Atraxi detection, he then made a psychic connection with Amy, taking on the form of her young self with the Doctor. The Doctor made his own psychic connection, making her think of Prisoner Zero, causing him to take on his own form. The Atraxi located and removed Prisoner Zero, but not before he once again warned the Doctor 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.