This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Odd Thomas
The bodachs from the Odd Thomas books. The idea that these shadowy creatures appear at scenes of violence and death is bad enough, but then you learn that they show up because watching people in torment is exciting. Not only is someone coming after you with a chainsaw, but there are things watching it happen and cheering it on because they feed off your fear. Yeesh.
The way they're depicted in the second graphic novel. They look a little like dogs, in the same way that a rotting corpse looks a little like the person it once belonged to.
That and the fact that as soon as they find out you can actually see them, they'll kill you.
The Senoculus in "Deeply Odd" which takes Odd's form with some disturbing differences like a Slasher Smile revealing razor teeth. It is invincible, supernaturally strong, and wants to suck out your soul... by kissing its victim.