Multi-form

The Multi-forms were a type of shapeshifter.

Biology
In their natural form, Multi-forms were eel-like, with long bodies, wide eyes and long fangs. They were able to change their appearance by establishing a psychic link with another being and reading their minds, putting them in a comatose state. With this, they would take the form 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. Though it would take years to connect to a individual, Multi-forms could live for millenia and could afford to do so. Besides specific individuals, Multi-forms could also turn into a liquid, allowing quick getaways.

Multi-forms were also able to create perception filters. (DW: The Eleventh Hour)

History
A Multi-form, known as Prisoner Zero had been captured 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, creating a psychic link with several people, leaving 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)