This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Queen of All Oni
Jade being tortured by Lung until she goes completely insane, and then her astral form melts.
At one point during said torture, her eyelidsdissolve.
Jade's PTSD from the above is starting to reach this level, particularly the Freak Out she has when Tohru accidentally recreates the situation — she reduces to an almost animalistic state, and ends up slashing Tohru in the face.
Though they're disposed of easily enough, the monsters in the Vault of Endless Night are pretty horrifying, especially when it's hinted that there are things far worse even farther down.
The scene where Jade brainwashes Viper and turns her into the Samurai General is pretty horrifying, and it's made even more so by the fact that the PTSD mentioned above has caused her to completely snap.
Apparently the cult that trained Blankman worshiped an entity implied to be the Slender Man (which Blankman apparently kills)!
Hak Foo laying waste to San Francisco with the Mini-Khan is played a lot more darkly than in canon.