This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Ink City
In Log #3, aka the Benson/Mordecai/Heloise Log, Mordecai gets "I told you so" rights for life. How does he earn those? Simple: he can't convince Benson that trustingthat sweet-looking little girl is a bad idea. Said sweet-looking Mad Scientist proceeds to violate Benson with a crowbar, cut Mordecai apart and examines his insides while he's still fully aware and staring into the cavity of his open chest, then shatters Benson's headjust to keep him from screaming. The log ends with her pulling out Mordecai's heart and watching him die. Her only regret? That his blood tastes like ink, which is the only thing leaving a bitter taste in her mouth after all she'd done.
Log #44 and itsaftermath, wherein Heloise takes Rigby and lobotomizes him — without any kind of anesthesia — and installs a mind-control chip so that she can force him to try and kill his best friend Mordecai. Rigby is fully aware the whole time and unable to do anything to stop himself.
And Benson, unaware of what's happened, kills Rigby with a chainsaw, then goes My God, What Have I Done? upon learning he hadn't destroyed an evil inkblot clone, but murdered the real Rigby.
The implied Awful Truth behind the Ink Monsters: that they're simply lacking a 'blueprint' to copy, unlike the characters who have arrived in the city.
Depending on how you interpret what happened on the boat, either the ink monsters are capable of mimicking the voices and appearances of everyone who's disappeared... or they are everyone who's disappeared, now in And I Must Scream mode.
The me1TD0WN Event was specifically designed with this in mind. Thrasher, who already wasn't having much of an easy time in the City, begins to mysteriously suffer And I Must Scream effects on his mind and body... until he Goes Mad From The Revelation and mutates into a form stuck halfway between alive and dead. While in this state he forcibly learns not only everything about his own pitiful existence, but everyone else in the City's as well.
Including the fact that his world/series no longer exists, putting a terrifying twist on being too good to last...
This climaxes in a broken-down, insane Thrasher declaring a Taking You with Meon the City itself and proceeds to wipe himself out of existence and put a nice big crack in the Ink Well fountain.
The Regular Show cast's incredible luck continues during the World Split when Benson, Rigby and Mordecai all become shambling ink monsters. It's hard to say what's worse: the fact that Benson and Rigby are eaten alive and come back wrong thanks to the damaged fountain, or that Mordecai's predicament comes about because he was right next to the fountain, got a stopgap healing from the Mayor, and slowly deteriorates.
In LaboCanon, the Warner siblings have not had an easy time of it... and as this discussion shows, the scariest part is how naturally they became what they've grown into. ...That, and how the Warners of Ink City could potentially becomesomuchWORSE.
When Megatron arrives, he's not too pleased with Carolineknowing his name. Nothing Starscream tells him about their alliance dissaudes him from tossing her around like a rag doll and torturing her. And since Optimus Prime had shown him Death Is Cheap by letting him kill him before, he smashes her underfoot in a public broadcast just to give everyone a taste of his power and show how little he values her life.
Optimus Prime dying at Megatron's hands is bad enough. Made all the worse by the fact he did this in front of Bumblebee and Aisling.
The Cosmic Rust plague. Not only does it hit the Transformers cast hard, but this strain got mutated so that it affects more than Cybertronians, meaning other robots — and just metal in general — are put at risk.