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 ...
And before that, Gabby having her backpack stolen after Alex ditches her on their walk to school; it's quite lucky all the thief was interested in was the backpack because if he had wanted to, he could have really hurt her.
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.
A trilogy of books featuring the Team at a summer camp got pretty dark pretty fast with the first book featuring art projects made by the kids being massacred, the second book having various sabotages occur on a bike trip (including someone having their brakes cut and only being able to stop by crashing) and the third book opening with Hector having gone missing during a game of capture the flag and being Alone with the Psycho, who constantly demands to know where "it" is, with Hector being obviously terrified throughout the whole ordeal.
One of the books features a scam involving fake bills being forced on the elderly in Hector's neighborhood and the crook of the mystery kidnapping Hector and abandoning him in a warehouse. Yes; Ghostwriter got a message to the rest of the team and the new girl based on a contest winner found and untied him but if they hadn't...
The book "Blackout!" is pretty freaky; some bank robbers inflict a Big Blackout on the city and not only is the team split up but Gabby and Tina end up in a hostage situation halfway across the city from anyone else.