This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Ghostwriter
Gooey Gus the Slime Monster, a purple monster with a melted face and a horrifyingly creepy voice.
Appropriately enough, the doll was designed by Tom Savini.
The arc with the toxic waste. Gabby was the youngest member of the team at the time and seeing her so listless...*shudder*
Alex gets the code name wrong while infiltrating a team of thieves in the first story arc. This results in a chase inside a warehouse. If any of the team members got caught ...
The first episode, where the characters don't understand what Ghostwriter is or how he works, attempting to answer him by speaking, the visuals of him writing more and more desperately combined with the music can make him seem quite spooky.