Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Star Trek: The Next Generation
Q:If you can't take a little bloody nose, maybe you ought to go back home and crawl under your bed. It's not safe out here. It's wondrous, with treasures to satiate desires both subtle and gross. But it's not for the timid.
Even creepier, in The Best of Both Worlds, when Picard is assimilated by the Borg, he still calls Riker Number One.
The Borg Queen is introduced as a talking, disembodied head and shoulders being lowered down into the rest of her body (shown being assembled in later appearances; apparently, her body's stored in pieces when not needed.) Part of the spine hangs from the head and wiggles around in the air until it's all put into place. The aroused look on her face in the moments after being put back together doesn't help.
The Return managed to make the Borg even creepier than we saw in the movies. Vicious assimilated dogs, a giant construction/weapon drone (with a spiderlike "scuttler" that emerged from it), pumping organic parts inside the cube, and a multi-bodied engineered drone (only the first body has a head; when Spock asks what it does, he's told "It feeds the tubes."). If it were actually filmed....Yipes!
First Contact makes it a point to show how frighting Borg assimilation is starting with the ensign getting injected with nano-probes and having his skin turn gray and implants sprout from under his skin to the Borg montage where we see sickbay converted in to an assimilation chamber with the implants being installed.
Several crew members still undergoing assimilation are seen to have missing body parts that are presumably to be replaced eventually with Borg equipment. There's an extremely disturbing closeup of someone who's eye has been removed and replaced with some sort of metal object, and his eyelid is caked with some kind of bloody discharge.
There's also the fact that Picard flat out executes a crewman taken. It's supposed to look like a Mercy Kill, but then Fridge Logic sets in: Picard himself was recovered, and we know that Seven of Nine was recovered after a lot longer, so there's no reason to believe that he couldn't have been saved later. What the Hell, Hero?
Do you really think they would have time to save him, or any crewman that was assimilated? Take him with, and he'd be a liability. Leave him alive to be assimilated, and he would be just one more enemy to fight later. Picard made the logical choice under the circumstances.
In their introductory episode, a drone beams into main engineering and scans the ship's computer. It takes two phaser blasts to kill it, but is immediately replaced by another drone, and when Worf fires again it's blocked by the Borg shields. The drone then starts seriously messing with the Enterprise systems while staring very creepily at Picard and crew.
In Parallels you see a desperate and half-mad Riker◊ begging for help from the alt-uni Enterprise. In his universe, the entire Federation has been overrun by Borg, and he is the only man shown on the bridge. He is so desperate to prevent himself from returning to his own dimension that he tries to kill Worf and by doing so endanger all of the multiverses in the process.
Worf is with him. You can see him run from his console to try and fix the one that blows while he's talking.
An exchange that, on paper, should have come off as cheesy:
Borg Collective: ...Your culture will adapt to service us. Captain Picard: Impossible. My culture is based on freedom and self-determination. Collective: Freedom is irrelevant. Self-determination is irrelevant. You must comply. Picard: We would rather die. Collective: Death is irrelevant.
Q's behavior when the Borg where first introduced. At first he seemingly sent them into Borg space out of child-like spite. But to just coldly brush off the real deaths of eighteen innocent people as a bloody nose.
It's an unsettling reminder that Q, for all his puckish pranks and amusements, is genuinely a threat on his own, and he is so far above the Federation on the food chain that the lives of a handful of Starfleet officers mean absolutely nothing to him.
Q in general could be seen as Nightmare Fuel. Though he does have humanity's best interest in mind, for the most part, he is still a jerkish reality-warping alien who will gladly toy with you (mentally and physically) for his own amusement. Not only will he wipe out entire civilizations out of pure boredom, but he can also alter the laws of physics with little-to-no effort. And that's not even getting into how dangerous he can be when he's angry.
In the best Q episodes, its impossible to know what his motives are, but every time he has claimed to be operating as an agent of the Continuum he has been ruthless. Willing to execute Amanda Rogers—after possibly having already executed her parents—or imprison a fellow Q in a comet for eternity, were just...wow.
Really, Q is very reminiscent of Trelane, the titular Squire of Gothos... except with far greater power, and without two benevolent parental figures to bring him under control. That makes him a lot scarier.
A popular piece of Fanon (which John de Lancie has voiced support for) is that Q's kid from Voyager is in fact Trelane, which means Q is the benevolent (well, less malevolent) parental figure.
"Conspiracy" may be Darker and Edgier, but it doesn't tread into this trope... until Picard and Riker blast off Remic's head and a big worm-like parasite emerges from the body.
The neural interface that plugs Barclay into the main computer in "The Nth Degree".
"Datalore", where Data says "I'm" and twitches his eye, after sending Lore into empty space.
Guinan is actually rather terrifying when she encounters a depowered Q in "Deja Q." She finds him and she is enjoying herself tremendously upon seeing him Brought Down to Normal, even stabbing him with a fork. And then, after he's been attacked by the Calamarain and screaming for help, her reaction is to look down upon him: "How the mighty have fallen." It's such a shift from her usual attitude.
The conversation that takes place between herself and Q in the episode "Q Who" outright says they've had dealings before, implies that she's not the slightest bit afraid of him and her posture and hand positions when Q makes to snap his fingers suggest that she herself has some kind of power that she can use to counter Q. Q even says something to the effect of "So that's the name you're calling yourself now".
The demon worms that Barclay sees inside the transporter beam are creepy enough to the audience, but to someone like him who already has a phobia of transporters, it becomes all the more horrifying as we watch the previously shy, loveable Barclay slowly go insane as he thinks his mind is slipping away. True, at the end he confronts his fear and learns that the worms are actually people trapped in the beam, but it's still scary to watch it all happen. Deanna probably had to put in some serious overtime to handle that one.
This episode also shows that people are fully conscious during transport. Imagine how scary it must be to be conscious while the transporter disassembles you, converts you to energy, and then reassembles you back into matter...at a target several hundred miles away.
Armus capturing Riker, and dragging him inside leaves us with this haunting image◊.
The Doomsday Machine mentioned for The Original Series? Another one shows up in the novel Vendetta. It's bigger, it's faster, it's angry, it's haunted by yandere ghosts!
The scene just before Data's possession in Masks. While a diagnostic is being run on him, he tries to express something to Geordi, but is incapable of finding the words to describe it. When Geordi asks him about it, he replies "I do not know" in a totally uncertain tone that comes as close to real fear as Data has ever come without an emotion chip. Then the kicker, in a spine-chilling twist on Data's character trademark of persistent, endearing, humorously blunt inquiries about human behavior and feeling:
Data: Geordi? What...does it feel like when a person is losing his mind?
Nagilum, from "Where Silence Has Lease". An immortal, nigh-omnipotent Elder Thing who, in the spirit of scientific inquiry and genuine curiosity, decides to study the phenomenon of death. In order to do so, it rips a great big hole in spacetime, traps the Enterprise therein, and makes with the empiricism. And it manifests as a giant face floating in the void◊.
"Is it also true that you have a limited existence?... you exist, then you cease to exist. You call it... death."
...Which immediately leads to a redshirt being killed by a forced heart attack.
No redshirt has ever died in quite the same way, either—going from calmly sitting at conn, to seizing with a terrified look on his face, and then falling pitifully to the deck in the fetal position, still looking terrified.
And let us not forget Picard and Riker calmly and matter-of-factly deciding to self-destruct the ship with all hands aboard rather then have them be killed one by one by Nagilum. The conversation Picard and his Number 1 have over how long to set the count down to the ship's destruction is chilling. It gets worse when Fridge Horror sets in and we remember that besides the crew, the Enterprise is populated by a couple hundred civilians, many of them children. One can't fault Picard and Riker for wanting to spare their crew from the horrible screaming death we saw the red shirt subjected to, but the way they just give up without really exploring any other options is unnerving.
What options? They may as well have been trying to arm-wrestle Galactus. If they fought, they would have been destroyed. Threatening to deny their captor the chance to poke and prod them, and being fully prepared to follow through on that threat, was their only way of actually escaping. You'll note that they weren't killed in the end.
They go to a ship that is the exact double of the Enterprise, only there's nobody aboard at all, and there are strange inhuman screams echoing throughout the ship. Then Riker finds Worf, who is freaking out a little, and they ask each other if they were making the screams. They weren't. Then they find the bridge, and every door leads to one of the other doors on the bridge, so they're stuck and are visibly shaken when they return to Enterprise.
Worf pretty much lost his shit on the bridge of the doppelganger ship. "THERE IS ONE BRIDGE. ONE RIKER, ONE BRIDGE GRAAHHH."
Watching Data, aka the Nicest of Nice Guys, calmlystabbing Troi in the shoulder over... and over... and over...
The first revealing of Data's head, in a cavern in San Fransisco.
Later on, time starts to go screwy and we get a glimpse of the energy-draining ghost things.
The Devidians, the race of aliens in question, were picked up for use in one of the "Weekly Episode" story arcs for Star Trek Online recently, in the spirit of Halloween. The Devidians in the actual TV episode are nothing compared to the ones in game - they are in absolutely massive numbers, as opposed to the handful seen on TV, and their base of operations is the poorly-maintained innards of a space station that, until recently, was uninhabited since the days of The Original Series. Worse still, from that point on, you'll occasionally see the lights flicker in the populated regions of the station, and a ghostly Devidian will run by, apparently unseen by everyone else but you...
The Next Generation also had an episode where a child's imaginary friend turned out to be an evil alien. So you have a cute little girl telling another cute little girl that if she won't play her way, she can just stay and die with all the others.
And the episode where the ship gets cleaned by an energy field slowly sweeping through it that will kill any living being it touches. Of course, Picard and some thieves get caught on the ship when the field is activated. (One isn't seen dying, but is certainly heard dying, screaming horribly.)
"Remember Me," the episode where Dr. Crusher gets trapped in a pocket universe may be far scarier than was intended. There's a particular kind of hopeless terror when the borders of reality itself are closing in on all sides. A New Scientist article predicting that this might be what it would actually look like when the universe ends does not help.
People start vanishing and no one except Dr. Crusher believes they were ever there, not even the ship's computer. At least when the pocket universe started collapsing you knew what was going on.
The collapsing universe part almost came as a relief. What would have been far worse was if Crusher had just been left there, completely alone on the Enterprise, with everyone she knew gone, no way of possibly running the ship by herself, and still having no idea what happened.
COMPUTER: "The universe is a spheroid region 705 meters in diameter." CREEPY.
"The Next Phase", where Geordi and Ro, who were trapped in the alternate phase of reality, dealt with a guy by kicking him out into space through a solid wall. They appeared at their own funeral as ghosts writhing in pain as the Applied Phlebotinum of the episode revealed them to the rest of the crew.
Applying Fridge Logic to this episode makes it even worse. It's established that the people who are out of phase aren't able to interact with matter. This means that the out of phase people don't need air, or else it would have been a very short episode. Given that they don't need air, we can surmise they probably aren't affected by changes in temperature or pressure. Now apply all this to the Romulan who got shoved through the hull of the ship, last seen drifting off into space unable to counter the momentum of the push that sent him through the hull. Instead of dying a relatively quick death from exposure to space, the poor bastard will instead drift through space until he finally dies of dehydration.
Apparently people out of phase are still affected by gravity and can feel pain. Add into this potential immortality and, well, he's going to have to drift by a sun or a star eventually...
At one point in the episode "Night Terrors," Dr. Crusher hallucinates that an entire morgue full of sheet-covered bodies are sitting up on their slabs. It's unspeakably unsettling.
That whole episode is made of nightmare fuel. Noteworthy is Counselor Troi's reoccuring psychedelic vision: A 3-dimensional expanding mass of green fog, where you can't see through it, but still hints at more beyond with the bright glow of a crescent moon - which you involuntarily fly toward... sounds like fun, until-
And this mantra drove the one Betazoid on the doomed ship catatonic... at least it was only an attempt to communicate.
One of the more subtly creepy moments is when Picard is sitting in his office, and the door chime starts sounding... over and over and over... and you don't know or see what's causing it. Keep in mind, this is before you really start to realize what's going on in the episode, so the fact that no one is activating the door chime is freaky as hell.
The entirety of the episode "Schisms," where the Enterprise crew are abducted and experimented on while they sleep by creatures from deep subspace. Particularly the scene where the abductees try to reconstruct their nightmares on the holodeck, ending with them standing around a creepy operating table in the dark with strange clicking and buzzing noises in the background. You can even see an abductee's hands climbing toward her face in horror as they get more and more accurate. Or perhaps the scene in sick bay where Riker learns that his arm has been severed and then reattached while he was asleep.
The fact that near the end of the episode, Riker turned a phaser on himself in order to find out what was real and what wasn't. He was willing to risk suicide in order to get the truth. The fact that Riker of all people was that desperate is terrifying.
The episode Identity Crisis, wherein Geordi (along with a few others) contracts a parasite and is transformed into a bizarre alien creature. And that's how that species reproduces...
The scene in the holodeck where Geordi is de-constructing the video footage shot on the original away mission, gradually removing all of the crew members that were present until there's one remaining shadow, and nothing causing it: the computer prediction of the shape that cast the shadow is a formless, faceless blob◊.
The "psych test" Wesley undertakes as part of the Starfleet entrance exam. Everything about the test is terrifying, and it's also notably a rare season 1 instance where the Creator's Pet does not come out on top.
And the fact that that was precisely the point of the test! Using your psychological profile to tailor-make the scenario that will be most difficult, and therefore most terrifying, for you. And they do this to every, single officer. Even Worf was still visibly shaken by his test, which presumably took place several years ago.
"Sub Rosa" has Dr. Crusher seduced by an energy being who claims to be an 800-year old human ghost. It does this by absorbing into her, and she reacts with visible ecstasy. The being ends up taking over her mind and trapping her on his planet, all while claiming to love her and only wanting to make her happy.
The moment where her grandmother's corpse sits up in its coffin during a lightning storm with demonic, glowing blue eyes.
Very fitting, as the script for that episode was based on a story written by Ann Rice.
In "The Child", Counselor Troi is forcibly impregnated in her sleep by a non-corporeal life form.
Those... creatures... from "Realm of Fear", those... worm-things, slowly coming into view in the middle of the transporter haze.
"The Most Toys" features what's basically a phaser on steroids that boils its victims from the inside out, giving them a few seconds of unimaginable pain before they die. And we actually see a full body shot of someone being killed by it!
Lore. When he first appears in "Datalore," he's vaguely creepy. Then you find out that he's a ruthless sociopath, and that's creepier. But it's during the scene when he kicks his deactivated brother in the head, twice, for no practical reason, that he becomes truly terrifying. He isn't merely pragmatically self-centered; he ENJOYS hurting people. And he's strong and fast enough to tear out your femur and stab you with it before you could scream.
Lore: "Are you prepared for the kind of death you've earned, little man?"
"Conundrum" presented the incredibly unsettling notion that the crew's mind could be wiped and the entire crew be turned against a technologically inferior civilization without even realizing it. Just the notion itself is Paranoia Fuel.
The episode Violations is terrifying enough to watch as a child, but understanding the literal Mind Rape implications of the telepathic attacks pulls it squarely into Adult Fear territory.
"The Game", especially when Wesley goes to talk to Picard about starting an investigation. You see him putting something down as Wesley enters, and then after he leaves, Picard turns around and picks up a copy of the game without a word.
Dark Page. If you never thought Lwaxana Troi could be scary...
TNG never got a Mirror Universe episode, but the book Dark Mirror gives it a go, and it's nasty. Troi as Mind Rapist and Picard as a murderous psychopath were bad enough, but then Good!Picard discovers that this universe perverted William Shakespeare into a twisted parody of the literature we know. He can't even bring himself to look at his antique copy of The Bible (presumably he wants to sleep sometime).
What about the episode where the Enterprise gets stuck in a time loop and keeps repeating the same thing over and over again? Knowing what was going to happen - that you're all going to keep doing this over and over again for the rest of eternity, and there's a good chance that whatever you're trying to prevent now, you probably did the same thing LAST TIME would be insanely creepy.
Which is presumably exactly what the crew of the Bozeman had been doing for 90 years, and would have been doing for the rest of time if not for Data.
The ending dialogue indicates that unlike the Enterprise, the Bozeman crew had no idea that they were trapped in time.
Picard: Perhaps you should beam aboard our ship. There's something we need to discuss...
Bozeman becomes a bit of a running joke, being mentioned or referenced in a number of episodes. The last we ever canonically hear about the ship, though, is in Star Trek: First Contact, when it's being ordered to attack a Borg cube. The good ship Bozeman spent 90 years in limbo only to be sent to it's likely death a few years later.
Moriarty's line about how he experienced "brief, terrifying moments of consciousness" while he was trapped in the computer memory banks.
Moriarty's fate as noted in Fridge Horror, since we later discover without regular maintenance holograms can eventually suffer glitches that threaten to destabilise the entire program. In "Ship in a Bottle", Moriarty and his lady love are trapped in a portable holodeck and are able to have their Happily Ever After... until the program starts to degrade! And they have no way of signalling the crew for help when that happens! You Bastards!
And more possible Nightmare Fuel, remember The Enterprise D was destroyed in Star Trek Generations and Moriarty's fate has never been revealed.
The Crystalline Entity. Yes, it may look like a giant space snowflake, but the thing is pretty damn terrifying when you think about it. You're living on a Federation colony, then one day this thing descends from the sky and begins consuming all life on the planet. It's practically impossible to evade, extremely durable, and there doesn't appear to be any way of tracking it or providing warning of its arrival. On the off chance you survive, the planet you're on will still have been left a barren wasteland. It can and will kill ANYTHING.
"Genesis" is pretty creepy in and of itself, but there is some disturbing Fridge Horror to the episode. Mid-way through the episode, Picard and Data end up finding the corpse of a crewman who had been killed at his post by another member of the de-evolved crew. When the episode ends, the cure has been applied to the ship and everything is back to normal, with little memory if any at all of what they were prior. So how many Enterprise crew members killed—and quite possibly ate—their fellow crew members in their de-evolved state, without even remembering it afterwards?
"Worf... open your mouth." What follows is perhaps the single-most disturbing injury any one of the main cast is shown to receive. Dr. Crusher gets acidic, paralyzing venom sprayed into her face, causing her to fall screaming to the floor, completely writhing in pain. It takes about four people to hold her down long enough to sedate her and Nurse Ogawa later states that she had to be put into stasis before the venom could paralyze her nervous system and that she would need reconstructive surgery. Thankfully, the episode does not have Picad and Data finding her stasis pod before she's healed.
"The Mind's Eye": Geordi is kidnapped by Romulans and subjected to Cold-Blooded Torture to become their Manchurian Agent. He is repeatedly shown a series of horrifying images sent directly to his visual cortex via his VISOR implants. Towards the end of the scene, we zoom in to his face as he screams in agony.
When Data and Riker examine the satellite in "The Neutral Zone," we get several nice views of rotting corpses whose cold sleep chambers had failed.
John Doe's injuries when he's first discovered in "Transfigurations" are nothing short of horrific, especially with part of his face torn off and his brain partially exposed!