Melanicus was a being which resembled the medieval conception of a demon.
Melanicus was a Kalichura, whose race lived in the Althrace system. After trying to conquer Althrace, he fled to another dimension. He came into contact with Catavolcus, a tyrant from earth in the third century, and persuaded the tyrant to summon him into Earth's dimension via a magic ceremony, and thus joined forces to form an alliance. He eventually took control of the Event Synthesizer, disrupting the flow of history and causing the Millennium Wars. He was eventually stopped by the Fifth Doctor and Sir Justin. (COMIC: The Tides of Time)