Nackles by Donald E. Westlake (F&SF, January 1964) begins with the narrator discussing the characteristics of gods, and whether Santa Claus is one, before he goes on to talk about his sister and brother-in-law. We learn that the latter assaulted his wife on one occasion, but was convinced by the narrator (with the help of a baseball bat) not to treat her like that again. Later on, however, the brother-in-law reverts to verbally and emotionally mistreating his wife and kids, eventually inventing the idea of a satanic anti-Santa, Nackles, to keep his three children out of sight and earshot—he tells the kids that Nackles doesn’t leave presents, but comes up from his underground tunnels to capture and eat children who have been bad. Frank also tells other fathers about his invention, so the idea spreads and belief in Nackles increases.
In the final section (spoiler) Frank’s behaviour becomes worse than usual one Christmas Eve—with the expected results for someone who behaves like a spoiled child.
There isn’t much of a story here, but it is a neat, well-developed idea, with a good last line from a well-known Xmas Song (“You’d better watch out”).1
*** (Good). 3,050 words. Internet Archive.
1. Santa Claus is Coming to Town (not the original, but a version I like) at 00:49.