This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Chronicles of Riddick
Riddick himself. Something about a remorseless killer that can see in the dark and says things like "Don't worry about what I do for you, worry about what I'll do to you." scares people. Not sure why.
The Ghost Drones from Dark Athena are arguably this, as well.
How about the Necromongers? Join them, and you experience unimaginable amounts of pain. Refuse them and die. Escape them, and they'll destroy your planet (Which they were just going to do anyway).
It gets worse. The fact that the Lord Marshal can manipulate souls strongly suggests that, in this Verse, the Necromongers are absolutely right about the Underverse and what happens to the souls of the dead. Which means that everyone who dies in the Riddick films, no matter how innocent, winds up in a place so abhorrent, yet enthralling, that merely glimpsing it has inspired millions to commit genocide.
Quite a bit of the first movie could be applied here, given the whole "Horde of monsters lurking in the darkness" bit. Sure, they could be repelled by light, but that only holds out so long... and you can only point lights in so many directions.