You may be looking for the novel.

Managra was a malevolent creature who used the Gallifreyan art of mimesis to infuse itself into other beings' bodies. Its name was an anagram of anagram, cleverly designed to keep people confused about its true identity. As with Gorgons, the victim lost themselves by looking directly at Managra. When they did so, they saw their own faces, jumbled like an anagram.

It used its victims to gain knowledge of its surroundings, and eventually wreak havoc. It thought of itself as "the devil". According to the Fourth Doctor, though, it was just "a mimic, an entity that copies what it sees, repeats what it hears".

Managra was an ancient enemy of Gallifrey whom Rassilon tossed into the time vortex. There he was thought to have been imprisoned forever. Since it depended upon finding a host through which to experience reality, Rassilon believed the time vortex was the one place it could never find a corporeal host.

Nevertheless, it found Francis Pearson one day and fused with him to gain the knowledge to defeat Earth. Ultimately, however, the Doctor used his mastery of mimesis against Managra and defeated it. (PROSE: Managra)