This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Underland Chronicles
Six-foot rats, giant spiders, flesh-eating fish, dozens of rats falling down a chasm to their death - and though it isn't described, we're told Gregor hears them hit rock bottom. And that's just the first book!
Other books have delights such as Gregor's arms getting infected by some kind of octopus-like creature, a giant whirlpool, a main character getting eaten in seconds by countless tiny mites, another character getting her nose nearly bit off, poisonous frogs, plants that will charm you so they can dismember you, the implication that the Jungle is a Genius Loci ("Does this place have a brain?" "Where do you think all the paths came from?"), near-dehydration, Hamnet's tragic past, the mouse holocaust, the Bane...
Twirltongue is a much more subtle form of this: she's very persuasive, and the person that she focuses her efforts on is theBane. By the The Marks of Secret, she's controlling his every move, from ordering nibbler genocide to having Regalia sacked, and she does it all without the Bane knowing it, driving him further into insanity for her own ends. Her death is also very gruesome: she oversteps her boundaries a bit, and the Bane realizes that she's been manipulating him and rips her head off.