The Forsaken were a species that induced fear in others, then fed on the fear, killing the other being in the process. They could also use the guise of other beings to trick them.
History
A member of the species landed on a remote island in Singapore during World War II in 1942 and killed several of the soldiers stationed there. It used the form of the Grim Reaper, but also used the guise of Captain Clive Freeman to trick the soldiers.
Its plan was to use the form of one of the soldiers and get onto a rescue boat sent to collect the soldiers and head to a more civilized part of Earth. It was eventually stopped by the Second Doctor, who tricked the Forsaken into feeding on the fear in itself, killing the Forsaken in the process. (AUDIO: The Forsaken)