This is based on opinion. Please don't list it on a work's trope example list.
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.
The dying world of Charn itself. The sky is dark, the sun is near the end of its life, and there is no life anywhere. The city is full of immense ruins. Oh, and there are images of rulers that start out kind and wise and slowly degenerate into cruel tyrants.
“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 half minute 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
Aslan walks willingly to his execution, surrounded and jeered by his enemies. The Witch, relishing every moment of this triumph, orders her enemy humiliated. And just to twist the knife, she tells him privately that once he's dead, she will exterminate his followers anyway.
Anytime the Witch turns some poor creature to stone. She even did it to a butterfly, a FREAKIN BUTTERFLY!
Despite the kickass armor Jadis wears to the battle, it doesn't take long for the viewer to realize she's wearing Aslan's mane as a part of it.
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 SeaSerpent. 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 insectoid 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.
During the scene where people are being sold into slavery, the people who aren't get sacrificed to the Green Mist. It covers the boats and they're gone without a trace.
Eustace goes into battle as a dragon and ends up getting stabbed by one of their allies, a man so insane he mistakes Eustace for an enemy. Eustace gets better thanks to Aslan, but by that point, he'd been turned into a strange creature, forced to re-learn how to do a lot of everyday stuff, and then forced himself to be brave enough to fight a Sea Serpent. I he died, he wouldn't even have been himself.