Saint Nick (The Feast)

"Saint Nick" was, according to the Second Doctor, an elemental force. It was "conjured" by several 17th century Londoners who believed in Christmas, the practice of which was forbidden by Cromwell. "Saint Nick" feasted on the believers' belief in Christmas, which was a powerful energy that could send him home. In order to save the humans, the Doctor was forced to kill the alien, an act which upset Polly. (PROSE: The Feast)