This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Young Dracula
"They're hanging from the ceiling..."
From Series 4 Episode 4, a handyman has fixed the school locker, and turns to leave, and it is revealed that Ryan is lying on top of the lockers, a psychotic look on his face, and he looks in the direction the handyman went, baring his fangs.
Boris's entire story arc, which depicts becoming evil as inevitable once the sixteenth birthday hits. Made worse by the look on Vlad's face when he realizes that Boris has turned. The entire thing just makes you really damn afraid for him. Of course, we know from the revival that it all works out, but still.
Vlad bleeding from the eyes, ears, nose and mouth!
Anything involving Elisabetta, particularly when her mouth opens unnaturally wide!