Shlimazel

Shlimazel was a bogeyman encountered in Feet of Clay when he tried to detain Angua and Cheery Littlebottom in the Undead bar Biers. She flipped a fluffy blanket over his head, causing him extreme existential uncertainty - he didn't know whether or not he existed. It is well known that children hide their heads under the blanket to escape from fear, so to hide the fear's head under a blanket is a very effective way of causing it to stop being frightening.

Shlimazel briefly tries to annoy Susan Sto Helit in Hogfather, but is repelled by her "Death Voice".

Annotation

Shlimazel, apropriately enough, is a Roundworld Yiddish word meaning "a very unlucky or inept person who fails at everything".