Multi-form

A Multi-form was a type of shapeshifter.

Biology
In their natural form, Multi-form were eel-like, with long bodies, wide eyes and long fangs, slightly resembling a viperfish. They were able to change their appearance by establishing a psychic link with another being and reading their minds; this put the other being in a comatose state. With this link, the multi-form could take the shape of what the being was dreaming about. They could even take the appearance of several individuals at once, but they would need to be touching and would still be one person, sharing one voice and one mind. Also, the multiple heads could only look in a single direction at the same time. Though it would take months to connect to a individual, Multi-form could afford to do due to their lifespan. Besides specific individuals, Multi-form could also turn into a liquid or sand, allowing for quick escapes.

Multi-forms were also known to have perception filters and lifespans stretching milennia (DW: The Eleventh Hour)

History
Multi-forms are stated to be present in the observation deck (Platform One) during the expansion of the sun in 5.5/Apple/26 (DW: The End of the World). Though, it did not appear that they were in their original form.

A Multi-form known as Prisoner Zero had been imprisoned by the Atraxi for unknown reasons. When a crack appeared in space, it was able to escape to Amy Pond's house, where it hid for 12 years. It hid a room with a perception filter, created a psychic link with several people which left them comatose.

Eventually, the Doctor returned to the Amy's house and discovered the hidden room. When Prisoner Zero tried to attack them, it caught the attention of the Atraxi, who threatened to incinerate the Earth. The Doctor and Amy attempted to track Prisoner Zero down, though their attempts to attract the attention of the Atraxi failed. Eventually, Amy and Rory Williams were able to follow it to the hospital, while the Doctor created a computer virus that would be spread worldwide.

When the virus activated, the Atraxi followed it to its source, which was a mobile phone held by the Doctor. Prisoner Zero tried to hide by using the memories of Amy to take the form of the Doctor. The Doctor thwarted this by stimulating Amy's recent memory of Prisoner Zero and forcing it to take on its own form, allowing the Atraxi to find and capture it. (DW: The Eleventh Hour)