This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Heavy Metal
Heavy Metal was R-rated for many good reasons, including two segments filled with this brand of nightmare fuel: "B-17", which features not only most of the bomber plane's crew being killed in big bloody messes to the tune of a driving heavy metal song, but then it has a a zombie plague, and the part in the "Captain Sternn" segment where a trial-witness gets increasingly angry whenever the prosecutor asks him questions — eventually turning into an Incredible Hulk-like thing.
Who could forget the expressions on the little girl's face whenever they cut to her while the Loc Nar is talking to her. Those faces are sheer terror. It almost looks painful to watch.
The Loc Nar is also unsettling. Its voice gives you shivers, as does the things it says.