Tales from the Darkside is one of the great, creepy anthology series from the 1980s.
Embedded below is the episode entitled "Seasons of Belief" from December of 1986, which stars E.G. Marshall.
The plot: Two kids who don't believe in Santa Claus demand a story on Christmas Eve. They get a different story, a horrific tale of the monstrous "Grither."
My kids were so freaked out by this episode that they still get the willies when I just mention "The Grither!"