This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Hogfather
Jonathan Teatime. He's creepy enough when he's just terrifying Assassins and nailing dogs to the ceiling with that cheerful Psychopathic Manchild smile, but as the plot goes on he starts manifesting very subtle supernatural powers...
Hogfather is also notable for thematic exploration of normal Nightmare Fuel — as an Eldritch Location starts fighting back against Teatime's crew, more and more of them start to see manifestations of their irrational childhood fears. Except that those fears are now real and quite capable of killing them.
Possibly the most horrifying of these nightmares is the Scissor Man, who could be easily described as a Velociraptor made out of scissors.
The TV adaptation made the ghost of Ma Lilywhite pants-wettingly terrifying.
"It only kills monsters". It's horrifying to read, simply because we're dealing with a human monster — and the children recognized that.
The fact that Discworld is apparently crawling with bogeymen, Scissor Men, bears that appear if you step on sidewalk cracks, and other childhood terrors, that adults can't see but their child victims can. Even if they're only there to scare kids to feed off their fears rather than to harm them physically, it's a horrifying thought to consider what the nights are like for children who don't have Death's granddaughter for a governess.
Back in Wyrd Sisters, it's mentioned that Nanny Ogg encourages her grandchildren to believe that monsters from the dawn of time lurk inside the big cistern in her laundry room. If Hogfather is any indication, they really do.