This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Shrek the Third
The dream that Shrek has after he "wakes up" from his other dream.
IT HAD A BABY HEAD ON A DONKEY'S BODY!
Shrek states that when just a kid, his dad tried to eat him. That's pretty frightening to think about Shrek having to have endured that.
The DVD menus, especially the main menu. A bunch of curtains open, a bunch of fake clouds descend on THOUSANDS of strings (looking eerily like marionettes), and all you see is the blank, gloomy-looking, huge andnot quite real-looking stage where Charming's play takes place. No characters, no voiceovers, just that enormous and clearly artificial stage, engulfed in darkness, with sinister music playing in the background. The submenus (for extras, language selection, etc.) sort of subvert this by having some of the main characters present, but the huge menu titles meant to look like suspended stage props are still kinda creepy.
It is pretty frightening just how close Charming was to a complete victory. He already had staged a successful coup d'état and was in control of the kingdom for a day or so. He literally missed killing Shrek by a hair and would have won the crown permanently. If he did that much damage in a day, what would he have done long term to the people of Far Far Away?
And to the princesses who rose against him. Especially Fiona, who was pregnant with Shrek's children. Who knows what Charming might have done to her and to their triplets?