Multi-form

A Multi-form was a type of shapeshifter.

Biology
In their natural form, Multi-forms 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 an individual, Multi-form could do so due to their long lifespan. Besides specific individuals, Multi-form could also turn into liquid or sand, allowing for quick escapes.

Multi-forms had perception filters and lifespans stretching thousands of years. (TV: The Eleventh Hour)

History
A Multi-form known as Prisoner Zero was imprisoned by the Atraxi. When a crack appeared in space, it escaped to Amy Pond's house in 1996, where it hid for twelve years in a room hidden by a perception filter. It created psychic links with several people which left them comatose.

The Eleventh Doctor returned to Amy's house in 2008 and discovered the hidden room. When Prisoner Zero tried to attack Amy and him, it caught the attention of the Atraxi, who threatened to incinerate the Earth. The Doctor and Amy tried to track down Prisoner Zero, though their attempts to attract the attention of the Atraxi failed. Amy and Rory Williams pursued it to Royal Leadworth 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 recapture it. (TV: The Eleventh Hour)

Multi-forms were said to be present on Platform One during the expansion of the sun in 5.5/Apple/26, though it did not appear that they were in their natural form. (TV: The End of the World)