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Nightmare Fuel: Justice League
How Martian Manhunter is tortured in "Secret Origins." It, quite frankly, looks like someone digging their fingernails inside someone's face. All bloodless, though. Even so, it's terrifying. Matters are made worse when he is essentially ABSORBED into a different Martian.
The Justice Lords' Superman's Psychotic Smirk after he killed President Luthor, all with creepy background music.
Also Batman saying "Do you smell something burning" right before a scene cut to where we see the burned and melted down remains of the oval office desk and Superman standing next to it which lets us know that yes he did blast Luthor.
What he says to Batman and Wonder Woman right after makes it a LOT worse. I'm great!
Lord Superman lobotomizes Doomsday. The matter-of-fact way in which he does it and the speed in which Doomsday goes from being relatively well-spoken to drooling are equally chilling.
In a later episode, the real Superman is actually driven by desperation to attempt the same maneuver, but due to Doomsday's Adaptive Ability, this no longer works. Thus three bits of Nightmare Fuel in one scene: 1) Superman can do that, 2) And he will if he's desperate enough, 3) And on some enemies, it still won't be enough.
Being an homage to the Cthulhu Mythos, "The Terror Beyond" is stuffed to the gills with it.
Start with Ixhthultu himself, a piscine Expy of Cthulhu, which appears as a monstrous skull-like apparition with flaming eyes and Combat Tentacles.
What happens to the King of Kaznia, who is poisoned and paralysed by Vandal Savage as part of his plan to take over the country. He's unable to move and is forced to watch his daughter get married to the man responsible for his predicament. Made worse, since we never see what happens to him, it's heavily implied that he most likely perished when the rail-gun was redirected to destroy the castle!
Which if that wasn't bad enough, also means that Batman of all people, just accidentally killed Princess Audrey's father with a big honking space-gun!
Well actually, when we saw the people running out of the castle, you can see one of the soldier's carrying someone dressed in purple. And as I recall, the king was wearing purple pajamas. So, the king didn't actually die in the explosion.
The look of horror on Flash's face, when he briefly got spaced.
Doctor Destiny and Ace of the Royal Flush gang are especially fun cases in this regard. Their common special ability: Mind Rape. Which is to say, they are in themselves Nightmare Fuel in episodes that use Nightmare Fuel as a plot point.
The ending where Doctor Destiny has gotten everybody but Martian Manhunter and Batman to go to sleep. They show each of the heroes going to sleep, while playing a toned-down music box in the background. Finally, you hear Doctor Destiny softly chuckling. Not an Evil Laugh, just a... chuckle. Worst of all, Superman goes to sleep, and Batman calls him moments later, trying too late to warn him. End of episode.
Penny (wife): John, please. John: You know, I never liked that name, it's so... ordinary, Especially for someone who has such big things in store. You know, a destiny. Oh, destiny! I like it: Dr. Destiny. What do you think? Penny: You're crazy. John: I think you're finally seeing the real me. *clothes turn into spooky robes* And now that I'm a doctor *face falls off revealing a skull*'' I think I'll perform some surgery.
Destiny looms over the Flash when he enters his dreams, and says these chilling lines.
Flash: I get it, I'm dreaming, all I have to do is close my eyes and I'll... *closes eyes nothing happens* Dr. Destiny: Wake up?
While you're inside the dream he traps you in, you can't wake up, and the torture the victim goes through causes their heart to accelerate to dangerous levels until it eventually gives out. Batman and the Martian Manhunter made every attempt they could to wake the others, and Martian Manhunter in the end resorted to entering the dreams to save the others, with nothing else working.
Hawkgirl's nightmare deserves mention. Green Lantern, the Flash, and Superman all have nightmares relating to their powers. Hawkgirl has the much more relatable fear of confined spaces, which Doctor Destiny exploits by burying her alive. The terrified screaming from the otherwise tough Hawkgirl doesn't help. She breaks down and cries in fear, begging for anyone to help while Supes, GL and J'onn are right outside the locked dream door unable to go through, an unsettling reaction she never quite matches later on in the series. Prompting a Heroic Willpower moment in GL (which is good, as his powers are driven by willpower).
The cannibalistic kids in Flash's nightmare? I know that they had nothing to do with the nightmare per se, but it was so out of nowhere... They got to be even scarier than the context of the scene. Good thing he has superspeed. Or not.
Superman accidentally destroying Metropolis and snapping Jimmy Olsen's spine in his nightmare should also count. Not to mention accidentally frying Lois with heat vision at the very beginning. Both horrifying and heartbreaking, especially given what we see of his reactions - of all the Leaguers, his is the most outwardly pronounced. After he accidentally kills Lois, we cut to his apartment where we see him screaming and flailing around as if in agony.
The entire plot itself you can't deny this is pretty much a cartoon adaption of Nightmare on Elm Street, that was a horror movie for adults, this is a cartoon for kids.
J'onn being tortured by the Nazis. We don't actually see what happens, but we can definitely hear the screams...
Well, actually, that was probably him faking it, since he replaced the torturer easily enough. Shapechanging, super strength, phasing... he's equipped for this. Doesn't make his screaming any less creepy. Seriously, something about J'onn's voice makes any screaming, real or faked, kind of scary.
Given that the torturer looks weak when the guards find him strapped to the table, this leads to the very real question of whether some of those screams might have been from J'onn torturing him?!
One of the scenes in the Justice Lords episode, "A Better World", has the League visit the alternate Arkham. Many villains, particularly Batman villains, are acting very much out of character. Then, you notice two small dots on all of their heads. Then, you recall what Alternate Superman did to Doomsday. Oh, and Scarface's dots are on the dummy, not on the ventriloquist.
And of course, the first person who is revealed to have been lobotomized by the Lord Superman is the Joker; who is now the secretary of Arkham Asylum. I dunno about anyone else, but hearing his voice and seeing his appearance to be so calm, almost serene, it really drew an unsettling feeling considering we're used to seeing him as a psychopathic, murdering clown. Who woulda thought one way for Joker to be scarier was to calm him down??
Flash's scene with Poison Ivy - Ivy's a real unpleasant fanatical piece of work, in the DCAU and without, but seeing her like that... content, calm, friendly, and humming as she cuts the the buds off of flowers... Seems just fundamentally wrong. Creepy, you might call it. Especially when you think about Ivy's obsession with plants—she nearly goes into conniptions if anyone so much as touches one of her plants. For her to go so far as to actually be "hurting" plants... it's so thoroughly not her. And when the head of the robot clone of Justice Lord Superman (It Makes Sense in Context) falls behind her, she just continues on with her work.
At the end of "For the Man Who Has Everything", Wonder Woman puts Mongul's Lotus-Eater Machine on him. Right before the episode ends, we get to hear painful screaming and horrible destruction, presumably all from Mongul's fantasy. And Superman wasn't the only one screaming.
This episode deserves mention for when the plant is put on Bruce - we get to relive the moment his parents were shot (in black and white too to give it a proper Film Noir feel), but in his dream his father fights the criminal and continues to punch him. But as Wonder Woman tears off the plant, we see Bruce's father be overpowered by the criminal, zoom in on a child Bruce's face turning more and more horrified as we finally hear a combination of a tearing sound and a gunshot when the plant is removed... it's truly horrifying. Made worse in that when the plant attaches to him, we see the only genuine, contented smile Batman ever has in the entirety of the Justice League run.
Wonder Woman falling victim to a No-Holds-Barred Beatdown by Mongul was pretty unsettling to watch. We have seen Wonder Woman take a beating on Justice League before but this episode in particular makes it worse than the others. For instance you had Mongul coming close to actually killing her. She had to crawl to Bruce to save him from the plant because she could hardly stand after the beating she took from Mongul.
The end of "Kid Stuff", in which Mordred gets rid of all adults. It's reversed when Mordred is tricked into turning into an adult himself, and his mother Morgan Le Faye notes that the spell giving him eternal youth is now broken, leaving him only with eternal life. As if simply thinking about the ramifications of this isn't enough, the last shot of the episode shows Morgan tenderly wiping the drool from the mouth of a glassy-eyed old man.
Really, the nightmares started from episode one. However narmy the Imperium may have been, its attack was anythingbut.
For that matter, there's a certain shock to seeing what appears to be an ordinary guard dog start walking straight up a wall as it transforms into something entirely different.
J'onn getting multiple large, writhing alien tentacles shoved violently under his skin (including the skin of his FACE) is pretty gruesome.
The episode "Dark Heart". A planet being devoured piece by piece, the inhabitants mostly being able to do nothing as they see the wave of machines sweeping towards them.
Darkseid describing what he's going to do to Superman while he's writing on the ground in pain from the Agony Matrix. Specifically, Darkseid pulls out a kryptonite knife and tells Superman that he's going to carve out his heart and place it on a pike as a trophy.
The portrayal of Darkseid in the DCAU is often seen as the definitive portrayal of his character (outside of comics) and for good reason: he is a terrifying character. Not just because he's the only one who can take Superman at his full power, but because he is relentlessly evil. Other villains had reasons that you could like them - at least Lex Luthor was suave, and at least The Joker had a happy attitude and comic timing. Darkseid is not that kind of villain: he is the absolute embodiment of evil.
To give a sense of how terrible Darkseid is, he is the only villain that Superman actively tries and wants to kill.
A rather small one, but the Justice League women being pitted together in Grudge Match. Huntress and Black Canary manage to free Shayera and Vixen pretty easily, but then they find out they have to fight Wonder Woman (who could match Superman in terms of power). Their reaction is entirely justified. During a rather brutal fight, a few of the League girls come frighteningly close to a bloody death. Hawkgirl almost smashing Vixen's head with her mace, for example.
Also, the way Wonder Woman totally over-powered all four Leaguers and took absolutely everything they threw at her like it was nothing. Then there's the way she was about to kill the badly battered Shayera and Vixen by crushing their skulls together. More terrifying is the fact that she was completely unaware of what she was doing and would never have remembered it had she been able to follow through.
The vision the Question sees while being TORTURED is pretty chilling.
The Question being tortured period was chilling.
He was missing for almost a week before Huntress tried to get Superman's help. Even then, nobody in the League noticed that he was missing.
Static's howl after getting sucked into a spatial rip was rather chilling, as was Terry's death by the Dee-Dees. More chilling was the fact that the entire universe was literally collapsing all around them and they were all completely helpless to stop it.
It's easy to forget exactly how powerful the Martian Manhunter is, since he spends so much time being The Aloner. Then Task Force X sneaks into the Watchtower, makes off with the secret weapon they've been sent for, mops the floor with several unexpected obstacles, and suddenly comes face to face with That One Boss: J'onn. And suddenly he's really creepy.
In "Flashpoint," Lex Luthor hijacks the Watchtower's Binary Fusion Generator, forcing it to fire on an (abandoned) Cadmus facility. When the gun goes off, the scene cuts to a peaceful suburban neighborhood...that suddenly goes very quiet as the sky starts to light up. Then the blast wave comes and nearly levels the city, followed by a mushroom cloud. Aside from the fact that, somehow, nobody (onscreen) is killed, and there's no residual radiation, the entire sequence is frighteningly reminiscent of a nuclear attack.
That's right, kids! Death from above, with no warning, no escape, and no explanation! It could happen to you before you finish reading TV Tropes!
The ending of "Hearts and Minds". Despero's army is turned into trees, all the while they are aware of this. Their faces as this happens tells everything you need to know. Espcially chilling one a close up of a face with face stuck in a scream, while only one eye is left open...
Galataea's Family-Unfriendly Death (Supergirl frying her with the reactor), as well as a good shot of her twitching corpse!
Being that she's a clone of Supergirl, she might have survived... still disturbing, though.
Seeing Amazo tear through the entire Justice League to get to Luthor. In space, he destroyed an entire fleet of Javelins and defeats nearly dozens of heroes including Green Lantern, Dr. Light, Captain Atom and Superman himself. In the sky, he goes through the second wave (Supergirl, Fire, Red Tornado, and Rocket Red) like tissue, during which he splits Red Tornado in half and destroys himnote During his first viewing of this episode, this troper was not aware of Red Tornado's background and thought he was human, making it worse. He doesn't even try in the last wave (Wonder Woman, Flash, Steel, and Ice). The thought of something that powerful existing is horrifying. Good thing he became an ally.
J'onn is a master of subtle nightmare fuel in this series, take his mental interrogation of one of the Thanagarians. Keep in mind, he couldn't normally read their minds due to mental defenses.
While generally amusing, the entirety of "Legends" has some extremely creepy subtext - indeed, the fact that the entire arc is so bright and sunny makes it all the more frightening when you put together the pieces. The ideal, idyllic 50's comic books fantasy world was created by a young child who had to experience his entire world go up in flames when a nuclear war broke out, was mutated by the fallout into some... inhuman... cancerous... thing, and was driven so insane that the only he could cope was by using his newfound psychic powers to create an eternal happy dream world where none of this ever happened.
Tala's screams as she's literally being used up in Luthor's attempt to revive Brainiac. (The show-runners even admitted they were unsettled by this, even after having toned it down considerably from the original recordings.)
Immediately before this, when Tala begs Lex to spare her for betraying him to Grodd, she tells him she's "a sick person". When she realizes he planned on using her up even before she betrayed him, he coldly replies, "I'm a sick person, too."
Grodd getting spaced in this episode was pretty disturbing, as well.
The reveal of the real world in "Legends" is chilling when you consider how close our world has come to that and that it could still end up as such.
There is a (relatively) minor one in Eclipsed with the soldier who is possessed at the beginning: this guy wakes up and learns that he killed his whole unit, with whom he seemed rather close. Especially since the last time we see him in the episode he's still held responsible for it.