You may be looking for The Darkness (The Well-Mannered War).

The Darkness was a parasite. According to the Sixth Doctor, it normally lived in space but occasionally landed on a planet to feed. It fed on the darkness in people's minds and twisted it out of proportion.

The Darkness attached itself to a Verulan spaceship. When the Fourth Doctor defeated the Verulans, the Darkness became loose in London and infected several people. The Sixth Doctor, Colonel Emily Chaudhry and Lieutenant Will Hoffman destroyed it before it could spread. (PROSE: The Terror of the Darkness)

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