Fearmonger

The Fearmonger was an alien entity which fed off fear and hatred.

Powers and abilities
The Fearmonger itself did not stir up fear, but chose hosts capable of stirring up large amounts of fear and stayed with them for long periods, moving on if the host died. In addition to feeding off of fear, it could unleash mental bursts of concentrated fear on attackers, rendering them permanently insane with fear. When not inhabiting a host, it was vulnerable to any metal that could ground it, as its body was made of electromagnetic energy.

History
According to some stories, an alien civilisation created several psychic entities (each one personifying an emotion such as fear, compassion, pride, and anger) to incite group emotions and thus unite their people. When the civilisation fell, the entities left for other worlds. The entity responsible for fear eventually made its way to Earth.

Ace jokingly claimed that the Fearmonger was "a Fearmongoid from the planet Fearmongus", and likewise the Doctor facetiously claimed that it was created by the "Jinmoti of Bozlen Two", a reference to Iain M. Banks' Consider Phlebas.

In 2002, the Fearmonger took advantage of the volatile political climate in the United Kingdom generated by the New Britannia Party and its leader, Sherilyn Harper. It chose Stephen Keyser and, later, Walter Jacobs as its hosts, their own fear of Harper leading them to believe that the entity was using her as its host. They made individual attempts to assassinate her, but the first one failed and the second was stopped by Seventh Doctor and Ace, who were on the Fearmonger's trail. The Doctor knew that killing the host would not kill the entity, and had constructed a force field generator that would destroy it without harming the host. He failed in his initial attempts to kill the Fearmonger, which continued to feed on the fear generated by Harper's rhetoric and the terrorist activities of the United Front, which eventually led to riots breaking out in London. While the Doctor stopped the riots, Ace came to believe that the Fearmonger had taken the Doctor as its new host. In fact, she was the Fearmonger's host, and while under its influence she threatened to kill the Doctor; however, he convinced her of the truth and she used the force field device on herself, destroying the entity. (AUDIO: The Fearmonger)