The idea of a sapient race too small to see in your own home that wants revenge.
On the other end of that equation, the microscopic civilization suffered the destruction of every city they ever built due to the mansion's inhabitants simply going about their lives.
The Depths of Cousin Fethry!
The majority of the episode takes place deep in the ocean where Everything Is Trying to Kill You, including the great pressure, lack of oxygen, volcanic vents, enormous worms, and a gigantic crustacean.
He also shows another rare instance of being a true threat when he throws Webby into the storming ocean. It's heavily implied that she would have drowned if Louie wasn't there to pull her back onto the boat.
The Town Where Everyone Was Nice!
Something about the Stepford Smiler townspeople's behavior feels pretty off, foreshadowing that they are not what they seem to be. The first time one pulls a Nightmare Face behind the character's backs is unnerving as heck.
The town's flower is a giant Man-Eating Plant attempting to devour the Duck family, and the townspeople were puppets on its vines. The plant has a very unsettling design with eyes at the end of its vines. Worse, it is intelligent enough to control realistic-looking puppets, even speaking a few sentences, as well as come up with a fake festival and invite people from abroad to it so that it can eat them!
Webby discovering and then showing Dewey and Louie than none of the villagers have feet and they're all offshoots of the vines is just as scary.
When the Ducks and the Caballeros seem to have defeated the plant and flying away from the town, one of the plant's "eyes" drop on the ground, sprouting a new flower...
Jormugandr, a serpent said to be so big that he encircles the world, actually exists, and Scrooge is all that holds him at bay...
If Dewey had told Young Donald and Della about the future, what exactly could have happened? Could Della have ended up facing an even worse fate than the one she already had? Would Donald be the one to vanish? Would something even worse happen? Honestly, it is for the best that the kid twins stopped him when they did.
Scrooge shows in this episode exactly how he's managed to live this long. He may have a soft spot for his family, but he left the Ghost of Christmas Past alone in time with no way out. He never had the intention of going back to get him either. He also showed no remorse for what happened to the ghost after he left. Scrooge may have forgiven him in the end, but he did not show any remorse for what he did. Do not mess with Scrooge McDuck, for he will make you pay for it.
Lost, desperate people in this universe might turn into large, hairy beasts named Wendigo, wandering the woods aimlessly and attacking people. Even a Christmas spirit isn't immune from becoming one.
In traditional lore, the Wendigo is turned by cannibalism. In this universe, it takes mere obsession and desperation to turn one into a Wendigo, which somehow makes it even more unnerving.
Whatever Happened to Della Duck?
Upon crashing on the moon, Della's helmet is cracked and she nearly suffocates before remembering Gyro's special gum.
Then there's the implication she had to sever her left leg from the crash, which is an excruciating nightmare to think about.
There's something ominous about Lieutenant Penumbra and General Lunaris's conversation at the end of the episode.
Penumbra: I can't believe you're just gonna let that Moon hater run free. You've gone Earth-soft.
Lunaris: That Earth dweller managed to defeat our greatest enemy like it was nothing. She could be useful.
Penumbra: Or dangerous. (to Della) Watch your back, Earth dweller.
Lunaris shows his true colors and shoots himself, saying Della had shot him and betrayed the Moonlanders. This leads the Moonlanders to agree to invade Earth in revenge.
Donald accidentally gets launched into space. He can't pilot the rocket and doesn't have the proper equipment with him. And it implies he's about to land on the Moon, in the middle of the Moonlanders planning their invasion. Worst of all his family thinks he's on a month long cruise. They probably won't even realize that he's missing before the month is up, and by then it's likely the trail will have gone cold.
Nothing Can Stop Della Duck!
Donald ends up landing in the moon and quickly runs out of oxygen but fortunately, he finds some Oxy-Chew. After a moment of relief, he's immediately arrested by the hostile Moonlanders.
Friendship Hates Magic!
At one point Webby looks extremely disfigured, with her eyeball popping out of its socket. While we quickly learn it's just make-up, it still looks very disturbing.
Violets spell seems to bring back Magica in her shadow form, to Lenas complete horror. And then she realizes the spirits are actually a manifestation of her own jealous feelings toward Webby moving on from their friendship, and has to admit that kind of darkness is inside her.
In addition, unlike The Incredible Hulk, Beaks remains himself and is completely in control of his actions while he is in his monstrous form. This means Beaks wanted to kill Fenton even before he turned into a monster and simply lacked the means to do so.
The Duck Knight Returns!
Jim Starling slowly becomes more unhinged over the course of the episode, as he gets more obsessed with forcing himself into the rebooted film. While it starts out funny, his behavior becomes concerning when he knocks out two security guards pursuing him before stuffing them in a storage closet, then does the same thing to Drake despite being offered an olive branch by him and laughs maniacally with a crazed look in his eyes. The climax is a sharp veer into dangerous territory: he accidentally sets the movie set on ablaze trying to take over the film, then refuses to let anyone leave. Anyone who tries nearly gets fried with an all-too-real prop gun, and he doesn't care one whit as the fire continues to grow around him.
Jim Starling: Jim Starling never cuts! I'll film this finale if it kills me, and everyone on this set!
He is actively trying to kill Drake. He shocks him, blows him up, and at one point has a chainsaw ready to cut him to pieces. If Drake weren't as tough and scrappy as he was he would have been dead.
The aforementioned chainsaw is also a chilling foreshadowing of whom he's going to become.
The final scene. Though everyone believes him missing or dead, Jim's hiding in the sewers, seething as he convinces himself that the whole mess was a conspiracy to steal his spotlight. All the while, his voice gets huskier and more deranged, the dye bleeding out of his costume. Then he turns to face the camera to reveal a rather unnerving Nightmare Face - eyes swirling with madness, teeth now sharp fangs, sporting a Slasher Smile as he begins cackling about embracing a Darker and Edgier role. But the crowning shot has to be his costume - the purple hues of Darkwing Duck are gone, leaving an all-too-familiar yellow, red, and black. We have just witnessed the birth of Negaduck.
Lunaris has been keeping tabs on Earth since long before Della crashed on the moon, and has extensive knowledge on Scrooge and family which he uses to threaten Donald. Worst of all are his plans for Huey, Dewey and Louie.
Lunaris: You see, most fools would target Scrooge McDuck. They don't realize the key to victory is to take the children out first, to break your spirits. Say goodbye to your precious Hubert, Dewford, and Llewelyn.
Even played for laughs, Donald stuck in the vent along with some scorpion/crustacean creature that keeps crawling after him will really get to anyone with claustrophobia.
Happy Birthday, Doofus Drake!
Just Doofus Drake in this episode, which cements him as a sadist. Can you blame Louie for not wanting to see him again, let alone attend his birthday party?
Boyd's existential crisis is...unsettling. Not to mention his eyes melt out of his skull at one point, although he switches to a new pair of eyes later on.
Even worse, Doofus implies that this isn't the first time this sort of thing has happened at one of his parties.
You can experience how Goldie feels when she ends up in a glass cage where Doofus imprisons her as his "Goldiemama". She even (flatly) claims it's even worse than being stuck in a dimension where imps jabbed her with a million splinters while they whispered to her about her biggest failures.
Nightmare on Killmotor-Hill!
Magica hasn't given up on Lena yet: she's been actively tormenting her by creating nightmares for her every time she falls asleep, and has been doing so for days. What's worse, is that there's nothing even remotely suggesting the outside influence, meaning that hadn't Magica exposed herself, she would keep doing that until Lena breaks. Said nightmares include Magica tempting Lena to come back to her (and when it doesn't work, hunting her down Hitman-style), everything cheerful and lighthearted turning nightmarish, full of black-purple color scheme, ending with Lena's biggest fear: becoming second Magica. And it almost works.
Just take a look at Magica herself at the end of the episode. She has been hiding for 6 months under McDuck's nose, undetected even by Gizmoduck. And while she may have survived on her own, her mental state is even worse now due to prolonged isolation, revenge obsession and lack of food - her eyes became ones of a completely unhinged person. Now imagine what will happen if she finds a way to get her powers back...
Lena turning into Magica. Just it alone is horrifying considering Lena's fears of becoming her aunt and just how uncomfortable it comes off as.
That's not even getting into the other transformations in early scenes: Lena's reflection getting Magica's eyes, growing demon wings during the first dream and by the end of it, turning green and looking just like Magica. Much to her horror.
While a funny moment, Lena mentions being trapped in the Shadow Realm for months and losing track of time during her silly song to the others. Let that sink in...
Much like the A Nightmare on Elm Street movies that the episode is named after, one of the first truly scary dream scenes happen in the high school, when the goofy Dewey mascot starts teleporting all over a distorted hallway that only Lena sees. It's pretty genuinely freaky.
The Golden Armory of Cornelius Coot!
Anyone who has issues with spiders will have a very hard time with this episode, as the kids wander through a mine completely infested with them, which keep crawling all over them, out from under their clothes, etc.
Louie becomes the sole survivor of the Timephoon. Near the end of the episode, every member of the McDuck family is getting thrown into random time periods of history, with no way to get back, unless there is someone to use the Timetub. In a span of several minutes, Louie gets to see everyone he loved vanish in front of him, until he's the only one left to fix everything, with pirates, ninjas and other treasure hunters coming at him. All because he wanted to get rich easily.His expression definitely highlights the situation he's in.
Imagine what Della must have felt like: Witnessing both Huey, Dewey, and Webby vanish right in front of her. While Webby isn't her biological daughter, it still adds a heavy dose of Adult Fear and the one thing that could have been on her mind was "Not again".
Unlike the other family members, Launchpad has been sent to the future, has witnessed the end of the world. Even if he found it neat, it's still a frightening thought. He may have even been to an alternate timeline during "Moonvasion" where the moonlanders win.
Remember Tootsie? The friendly, little Triceratops who was Bubba's pet in the original series? Well, she is none of those here. She is now bigger, stronger and overall ferocious, attempting to trample anything (or anyone) that gets in her way.
Still being a new parent, Della's methods of grounding are unintentionally sadistic. She has Gyro reprogram the DT-87 (which has a history of turning evil) to fire lasers at any attempts to escape as well as cast laser cages over the exits, records rather cheery lectures about ethics that almost sound condescending and, to top it all off, when family members call Louie to tell him how much he'd love where they are, she stops Huey from describing the scent of the Cherry Pep river to his brother because "He hasn't earned it". It is for his own good, but just maybe she went ever so slightly too far?
Glomgold and his "family" attacking McDuck Manor. There's no comedy to it (with the exception of Magica kicking Duckworth out with an amulet) and it's played off like a full blown invasion you'd expect to see in a finale. And while it was an Epic Fail on Glomgold's plan who didn't even bother checking if the family were in the manor, but had they been, it would have been an example of Glomgold either actually being a threat, or worse, winning. It definitely proved that Glomgold was right: When they all worked together, they would have taken down Scrooge and the family!
Add to the fact that when they realize Louie is the only one home, Magica is a-okay with killing him.
The Richest Duck in the World
Whoever becomes the richest duck in the world gets chased by the Bombie, essentially a Revenant Zombie that is indestructible and impossible to stop. His appearance is already quite scary with his hulking physique, scarred grey skin and color-inverted eyes, but what makes him really terrifying is how he No Sells Louie's and Owlson's attempts to stop him.
In his attempt to stop the Bombie, Louie shuts down the satellites around Earth. At the end of the episode it's revealed some of them were defense satellites, and shutting them down makes Earth vulnerable to an Alien Invasion.
And as the last scene shows, Della and Dewey receive a message from Penumbra: with the defense satellites down, General Lunaris has decided to launch the invasion now!
Della describes how before she met Penumbra, she was so desperate for companionship she spent three weeks having a staring contest with her own reflection, and ever since can't stop seeing that reflection as actually being another person.
At least two of the Gyro clones (or possibly the original Gyro, they lost track during the cloning) are outright killed.
Lunaris reveals that he seeks to make the Earth orbit the Moon, something he succeeds in. Following a Time Skip of indeterminate length, this is in the process of freezing the Earth.
When Lunaris's ship is damaged enough to thwart his orbit plan, he flies it into space with the intention of annihilating the planet with a Colony Drop, uncaring of the fact that his own people are still on it.
Lunaris: If the people of Earth won't live in fear of the Moon, then they'll DIE in fear of it!
Just when it seems that all of Lunaris' turrets are destroyed and the Ducks are in a position to take out his engine, he reveals a ton more turrets on the backside and powers up the engine. This causes the Ducks' ship to start falling apart, and they almost end up pulling a Heroic Sacrifice.
Even if he deserved it, the fact that Lunaris is now stuck inside his broken-down ship for all eternity, with the fact that it's perpetually revolving around the Earth like a moon being the definition of an Ironic Hell for him.
One shark is also stuck on the ship and unlike Lunaris, it didn't deserve that fate.
The final scene of the episode sets up the main villains for next season: F.O.W.L. Complete with an imposingly lit meeting room and the reveal that a few villains believed to be dead are still alive, and several supporting characters were Evil All Along, with the High Command being none other than Scrooge's own board of directors. Having nearly lost Earth to Lunaris, F.O.W.L. decides the Duck-McDuck family now needs to be eliminated.
Bradford Buzzard: If the Ducks want an adventure, we'll give them their last.