The entire episode could count as this; it features everyone's worst fears coming to life and attacking them. For young minds watching this episode in the dark, it is one of the scariest things you can possibly ever watch. Just wait until you see the vacuum cleaner.
Realistic or not, the kids in the audience might relate to some of the boys' fears:
Doofus' nightmare is at first somewhat laughable, involving a banana turning into a monster to prevent him from eating it, but then it quickly gets a LOT less funny, when Doofus' school bully Bully Beagle shows up and threatens him. When Scrooge finds him, Doofus cries about how much he hates it when Bully picks on him.
Another relatable fear is Huey admitting that he's scared of a video game villain who's one of the illusions terrorizing them. Many of us can remember being afraid of fictional monsters and villains as young children, but we never admitted it because we were afraid our friends would think we were wimps. Hell, video game villains can arguably be worse than ones from TV or movies because you effectively have to confront them yourself as part of the game, even games meant for younger or all-ages audiences. Imagine being a kid and being confronted by, say, Dracula from Castlevania, Ganon from The Legend of Zelda, or Dark Matter from Kirby.
Even the banana monster is somewhat scary, with its creepy green eyes, red slithering tongue, and scowling voice provided by none other than Frank Welker which sounds surprisingly similar to Megatron.
And then there is the teacher of the nephews, an ugly looking (anthropomorphic) vulture with crazy eyes and talon-like finger nails.
Donald while he's possessed by the pharaoh. Him coldly ordering Scrooge and his own nephews to be tied up and eaten alive by vultures circling overhead without a second thought is bad enough, but hearing the orders coming from a beloved animated character like Donald Duck manages to make the whole scene even more disturbing.
The episode is creepy no doubt, but there wasn't anything in it that sent it into complete nightmare territory... at least up until the point where the shadow of a skeleton comes out and starts strangling Uncle Scrooge. Then he goes limp. Granted, we knew he wasn't dead (he's the hero and this is a Disney story), but the visuals could have fooled the non-Genre Savvy viewer.
The scene where Mrs. Beakley and Webby are making the bed and suddenly discover the outline of a person under the sheet is genuinely hair-raising since at first there is no sound or movement, or anything to indicate something like that is about to happen. It's just...there.
The Golden Death. As you get older after watching this one, the Fridge Horror truly sets in. For example, there's the scene in which Scrooge's mansion is covered and everyone inside is transformed into a gold statue. It moves so quick that if you're caught unprepared: one second you could be minding your own business and the next you'll never know anything again. And even if you do manage to see it coming and try to escape, you'll get less and less space to run to since it will cover the entire world, and after touching something that's been changed you'll also be turned to gold within seconds. Even taking to the air would ultimately be fruitless since there's only so long you could stay up before your plane ran out of fuel and forced you to land, sending you into oblivion.
Even more fridge horror: the gold wound up covering nearly the entire world. This means that wherever they were Donald and Darkwing Duck wound up getting turned into statues wherever they were as well. They wouldn't even have known what was going on, just that there's a gold wave chasing them and turning everything in its path.
Not to mention that the Golden Death nearly overtakes the entire world before it's stopped. Scrooge ultimately has to throw the goose because he can't get there in time. That means that everyone who the viewer has been getting to know and love for the entire show was effectively dead, even if it was only for a moment or two.