KrampusKlaus — Christiana Miller

When Gina and Bobby wake up at midnight on Christmas Eve, they catch Santa in the middle of his present deliveries -- but he's unlike any Santa they've ever imagined. He's short-tempered, rude and wouldn't know jolly if it ran him over with a sleigh. It turns out that Santa has been cursed. Gina and Bobby don't figure out a way to save Santa, he'll turn into the demonic Krampus, (Hell's official Punisher and Tormentor of Naughty Children), forever.

Note: This is a short story, originally published in the Naughty or Nice anthology.