Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: The Chronicles of Narnia
The illustrations of Tash.
"Always winter... and never Christmas!"
That is, when this is expanded, the idea of eternally being witheld from a ceremony of hope is horrifying.
Many of Jadis's actions are Nightmare Fuel, but the fact that she exterminated all life on her original homeworld (other than herself, of course) with the Deplorable Word, and displayed no sign of remorse or regret over it, might just be the most horrifying.
“Fools!” said the man, stamping his foot with rage. “That is the sort of talk that brought me here, and I'd better have been drowned or never born. Do you hear what I say? This is where dreams—dreams, do you understand, come to life, come real. Not daydreams: dreams.” There was about half a minute's silence and then, with a great clatter of armour, the whole crew were tumbling down the main hatch as quick as they could and flinging themselves on the oars to row as they had never rowed before; and Drinian was swinging round the tiller, and the boatswain was giving out the quickest stroke that had ever been heard at sea. For it had taken everyone just that halfminute to remember certain dreams they had had—dreams that make you afraid of going to sleep again—and to realize what it would mean to land on a country where dreams come true.
The Lion The Witch and The Wardrobe
The whole scene just before Jadis kills Aslan.
Anytime the Witch turns some poor creature to stone. She even did it to a butterfly, a FREAKIN BUTTERFLY!
The hag (in the scene where they're trying to bring back the White Witch).
The minotaur being crushed under the gate and all the other Narnians being trapped.
Voyage of the Dawn Treader
The Sea Serpent. At first it just seems like a regular (if mildly scary) Sea Serpent but then partway through the battle... it opens up to reveal hundred and hundreds of legs, all writhing around. It's effectivly combining several Primal Fears: fear of drowning at sea, a huge foe, and a fear of insects.
The Dark Island. In the book it's a vaguely creepy location that is never mentioned after it is visited. In the movie, it is a Genius LociEldritch Abomination that wants nothing more than to swallow up the whole world with its own fears, and it is never explained where it came from or even why it exists, and it created the aforementioned Sea Serpent.