Sandman finally being overpowered by Pitch after a ferocious battle where the two are pretty evenly-matched. Just the look on his face as he's watching himself become consumed by the black sand, and no one can help him. Also doubles as a Tear Jerker towards the end.
Jack trying desperately to come help him, flying as fast as he possibly can, makes it all the worse. He originally didn't even want to be part of this, and now...
There's a shot towards the end of the climax in which Pitch's Nightmare sand has formed into a giant wave that is coming from every side, moving towards the heroes and the town like a flood.
Pitch's lair could be considered Nightmare Fuel in an atmospheric sense. Located in a hole in the ground, it is a drab, dull and colorless environment that is absolutely joyless compared to all the other environments in the film. And Pitch could be lurking in any shadow...
The entire scene in Pitch's lair counts as Nightmare Fuel in and of itself.
Sure, it may have ended in a bit of a Narm moment since Jack interrupted him with a snowball, but Pitch's Slasher Smile when he's asking Jamie if he believes in the Boogieman is really creepy.
Later, Pitch does it again when he dissolves into shadows and rises up behind Jack about to strike him down with his scythe with that same expression still on his face.
Pitch does a lot of Slasher Smiles throughout the film - it almost gets progressively creepier as he gets stronger.
It's not nightmare, but it heads in the same direction when you see what happens to the Guardians as they return to their mortal forms more and more. Bunnymund turns into a normal bunny, Tooth Fairy's wings moult and fail, and you can actively see her trying and failing to fly, and Santa is the worst of all, as he is no longer strong and powerful, but a tired old man. You can almost hear those bones creak.