This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Old Kingdom
Considering that the series involves undead spirits hijacking and apparently sometimes reshaping dead bodies as standard Mooks for the various villains, this comes as standard.
One particularly worth mentioning is a Dead Hand specifically described as having an elongated jaw and running towards a little boy who's to terrified to run away and just screams... until the jaws snap shut and the screams stop. Lirael's reaction, unsurprisingly, consists of Berserker Tears and her hacking the Dead Hand to pieces.
The first time we see Mogget unbound is possibly one of the most frightening things in the series, especially the sheer glee he takes in his description of how he's going to torture her to death...
Kerrigor lives and breathes this. Metaphorically speaking.