How about near the end of "Maid Of Honor", when Vandal Savage who was in ground zero for an asteroid hitting the Kasnian royal palace was in, confirms his immortality by breaking out of the rubble, still healing from the blastwith him still resetting his bones and screaming in a mixture of rage and indescribable pain?
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.
How about Joker revealing Ace's origin (and her powers, she can alter your perception of reality by looking at you), while her eyes stare at you.
The ending of "Only A Dream, Part 1" 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.
The scariest part of that definitely being Superman going to sleep, and then Batman calling him moments later, trying too late to warn him. End of episode. *Shiver*. It doesn't help that Destiny's one of the earliest villains to actually kill someone in a way we can see onscreen - he "operates" on his wife, and she later dies of a nightmare induced heart-attack.
Penny (wife): You're crazy, John. John: You know, I never liked that name, it's so ordinary, I'm someone with a destiny. Oh, I like that: Dr. Destiny. *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.
One thing that always creeps me out, is how Destiny looms over the Flash when he enters his dreams particularly during these 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?
Of course, after the Nightmare FuelNeil Gaiman got out of this character in the climax of the first Sandman miniseries (back when it was primarily a horror comic), nothing he does in this show can really compare...but some of it's pretty bad anyway.
Then there's the scene where Ace shows the Joker that no matter how crazy you are, there's always an even lower depth to plumb...
Hell yeah. I consider that a CMOA on her part. Seeing the Joker Hoist by His Own Petard was simply priceless.
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.
Not just screaming. 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).
Hawkgirl's nightmare is a bit of Fridge Brilliance by the writers. Flash and GL got their powers as adults, and Superman grew into his powers in an isolated farming town full of non-powered humans, so, for them, their powers have always made them different and slihtly alien from others, and so, they're bound to be a little afraid of them. However, Hawkgirl grew up on a planet were everyone was super-strong and had wings, and so sees herself as perfectly normal; so any phobias or neuroses she has will not be connected to her powers, because, as far as she's concerned, she doesn't have any.
Speaking of this episode, how about 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 accidentaly 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.
What about what Doctor Destiny did to his wife? She left him to be with another man, but upon getting his powers, he paid her a visit and unveiled his costumed, skull-faced appearance. The last thing he says to her is, "And now that I'm a doctor, I think I'll perform some surgery." Whatever he does to her, she is left screaming in a sleep no one can wake her from. She dies not too long afterwards.
The "Legends" last part.
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.
Either that, or stopping the evil plan that had resulted in the apocalypse in the first place resulted in the actual death of the mainstream and canon Savage... which is unsettling in and of itself.
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.
This troper isn't quite sure whether he should be amused or what by the fact that Scarface had the dots on his head rather than the Ventriloquist.
As much as Dissociative Identity Disorder doesn't really work this way, this does make the in-universe pathology consistent. Wesker believes Scarface has been lobotomized and acts accordingly because he can't act any other way. Which kind of makes the whole thing more depressing, really.
Then again, Wesker never did like what Mr. Scarface did to him to begin with (can you have an abusive relationship with yourself?). Maybe he's happier that way.
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??
Also 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 is even worse in the comic as we actually see Mongul's hallucination.
And what is that hallucination, then?
In the original comic story Mongul hallucinates that he swatted the Black Mercy away before it could grab him, while zapping Robin (Jason Todd, for the record) to the bone. Then he tore off Superman's head (laughing at how the eyes rolled for long moments), put it on a pike as his bloody standard, and conquered the Earth. Then he rebuilt War World and started conquering the galaxy, obliterating worlds on a whim, and with a panel where the great powers are lining up to kiss his fingertips... including a Guardian of the Universe, meaning he had defeated the entire Green Lantern Corps. Last line, the same Arc Words for all the other "perfect" imaginary worlds: "He is content."
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. This one manages to outdo the comic version (where nothing happened to Thomas, the mugger was arrested, and Batman even imagined himself married with a child with Batwoman - who, by then, wasn't yet a lesbian).
Made worse in that when the plant got onto him, we see the only genuine, contented smile Batman ever has in the entirety of the Justice League run.
While it doesn't stand out as the others 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 cause 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.
However, there's also some fridge horror to go along with the episode. Sure, no one's supposed to die in the DCAU, but the pilots of commercial airliners and the captains of cruise ships, as well as most people driving cars, are adults. Do the math when they disappear and those vehicles are in motion.
Even more nightmares come about when you think about infants or toddlers now left alone once their parents disappear...
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" was this for me, as well as doubling as a CMoA for the Atom . It gave me the sight of 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. And this is a kids' show people.
