This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Dragon Age: Origins
The Broodmother and the entire explanation given for it. First off, just the appearance is grotesque. Then, the explanation. To elaborate: They take some poor group of people, and make one of the females eat most of the rest and taint her. Those that are left over are some of the other females, because seeing that makes them break more easily when it's their turn. The tainting also involves 'bile and blood' being poured down the subject's throat and... violation that turns them into utter Body Horror, and over the process of a week, they become the Broodmother, who turns out more darkspawn.
Fifth day, they return and it's another girl's turn
Sixth day, her screams we hear in our dreams
Seventh day, she grew as in her mouth they spew
Eighth day, we hated as she is violated
Ninth day, she grins and devours her kin
Now she does feast, for she's become the beast.
This isn't the worst part. Branka deliberately allowed her female retinue to be infected, so that their Darkspawn progeny would provide her with a limitless supply of test subjects.
That's not the worst part, either. Hespith, who narrates all of this to you? She was Branka's lover. Branka left her behind anyway, and Hespith's now the last woman alive from the entirety of Branka's noble house. Hespith has seen every last one before her die or corrupted, and she knows what's coming next for her. And she knows that it's happening to her because she fell in love with the wrong person.
That's STILL not the worst part! You remember the Dwarf Noble origin? You're sent to be locked in the deep roads. If you're female, then just imagine what would happen if, say, you didn't get rescued by Duncan? You would have become a broodmother. Of course, this doubles with Fridge Horror.
It is presumed that the events of all origins took place in a game universe, with the player's choice only defining which one of the potential main characters Duncan will recruit. In the Dwarf Noble origin, luckily, it seems it defaults to a male character, which spares him the grisly fate.
To find this you have to walk right into the Dead Trenches. A city that used to be a Necropolis, overrun by thousands of darkspawn and is still haunted by the ghosts of ancient Dwarf warriors, the place itself looks and feels like hell. The Broodmother is just the climax of a slow escalation of creepiness that begins when you first enter the place and see the massive army of darkspawn marching under you, and watch the Archdemon himself fly overhead, shrieking horribly.
The orphanage in the Elven Alienage. A bunch of ripped-apart children's corpses, blood splatters, and plenty of mysterious voices spread through the place give an unholy idea of what's been going on there. Hell, going through there makes one feel like Alma is going to come skipping around the corner any second...
Take a look at the map. You exit through the back... into the front. Nothing on either map suggests a loop around, and this is AFTER you've 'cleansed' the place.
And don't forget the fate of poor Ser Otto, the blind Templar who helps you throughout the whole thing.
The Blood Wound spell, which boils your target's blood from inside their very veins. They just stand there twitching when it works...
Similar to the above "Blood Wound" spell, "Walking Bomb" turns your enemies into mobile explosives, and then into fountains of blood. In that order. Worse yet is "Virulent Walking Bomb," which is functionally identical... unless the explosion injures a target, in which case there's a chance the spell's effect will infect the target. And the worst part? Unlike the above "Blood Wound" and below "Blood Control" spells, which (by virtue of being blood magic) are forbidden by both national and church law, the Walking Bomb spells fall under spirit magic, and therefore are LEGAL!
There is a Blood Magic spell which can control any living thing with as a marionette by moving its blood. And the victim will suffer horrendous body damage in the case of resistance.
Even more terrifying after they've used it on your party once, so you know what's going to happen when combat suddenly stops and everyone around you gets friendly... there's nothing you can do before you and your party are suddenly frozen in place and turned into gushing fountains of blood that all flows toward the enemy blood mage. Anyone who survives to fight will be near death, while the blood mage is replenished.
The entire sequence that plays out if you decide to kill Connor instead of entering the Fade. It bounces back and forth between Nightmare Fuel and Tear Jerker and doesn't know when to stop.
The Tranquil. Despite being benevolent - as benevolent as a soulless walking mannequin can be, anyway - they make you want to hug your pillow and cry, just to remind yourself that you still can.
Particularly the ones in the "Broken Circle" quest, the ones who barely even have the capacity to be afraid of the demonic abominations rampaging around the tower. This is especially true of the Tranquil who calmly tells you not to go into the tower's stockroom because it's in "no fit state." During the conversation, he says that he attempted to get to safety, but found Wynne's barrier in his way, and, rather than attempt to get the attention of the mage who conjured it, chose to go back into the stockroom. (Wynne, who comes with you to cleanse the place, states that she would have let him through if he'd said something.)
During the "Broken Circle Quest", in the room where the last statue for "Watchguard of the Reaching" quest is, you're faced by an abomination, 3 undead and possibly 3 shades. If you pay enough attention during the first few moments of the fight, you can see 3 or 4 mages bind in a magical barrier. If you kill the abomination too slowly, they will turn to shades one by one. However, if you get to kill the abomination fast enough to prevent them from being turned, you will notice that they are not mages. They are Tranquil. This indicates that the Tranquil are not immune from demonic possession, even after sacrificing their emotions and their spell casting abilities, which means they are turned into Tranquil for nothing. Add the fact that normally, Tranquil are seen as undesirable by demons, and yet the ones in the Circle decided to possess them. Gives you an idea how bad the situation is. Either that, or the Tranquil are being sacrificed by the abomination to summon the shades. It's disturbing either way.
This is mitigated somewhat by the fact that regardless of their lack of emotions, the Tranquil are not just Empty Shells and still have free will. Many of the ones you talk to agree that making themselves useful to the Chantry and living peacefully is more preferable than falling to demonic possession (which is a real threat amongst mages), meaning they still have enough motivation and purpose to do things beyond mindless obedience. One of them, Owain, even points out that his lack of emotions doesn't make him any less a person than the party he's talking to. This is further proven by the presence of Pharamond in the novel Dragon Age Asunder, a Tranquil researcher who was sent by Justinia V to study the Rite of Tranquility and perhaps find a way to reverse the process that would allow mages to keep their minds without their power.
However, Dragon Age II reveals firsthand what being Tranquil is like, seeing the world devoid of all colour and music. Having briefly regained his sense of self due to Justice's presence and feeling himself slipping once more, Karl immediately begs Anders to kill him while he's still himself.
Fortunately, Inquisition reveals that Tranquility is not irreversible: The Seekers are all made Tranquil and restored by communing with Spirits of Faith, a ritual that renders them immune to further possession and grants them Anti-Magic powers similar to Templars.
Unfortunately, all of those Ocularums scattered in the world were made out of Tranquil skulls by Venatori. The companion reactions to this (It's in a locked house in Redcliffe in Inquisition) are sad.
The darkspawn are Nightmare Fuel and Squick already, but one survivor's description of what happened at Ostagar is pure undiluted horror. Darkspawn everywhere, captives being eaten alive, the very ground literally rotting underfoot like fetid meat; it's like Mordor, but even worse. You don't get to see it, but the veteran's account is more than enough to give the player nightmares. The Stone Prisoner DLC, however, gives you a firsthand look at what happens to towns that fall to the darkspawn.
The altar in one of the houses in Haven: coated with years of dried blood and JUST the right size for an infant or toddler.
Redcliffe Village becomes a ghost town if you leave during the quest to prepare it for the impending little Zombie Apocalypse. Nothing Is Scarier indeed. No wait, the really scary part is seeing some of the now undead villagers later in Redcliffe Castle.
While on the subject of Redcliffe, one militia member's description of what happened to his friend is this trope. One of the devouring corpses, a dead body possessed by a demon of hunger, got on him and started eating his face. He screamed and tried to push it off, but couldn't...
Also in Redcliffe, kids speaking with regular adult voices creep me out. Kids speaking with otherworldly demonic booming voices with a superimposed kid voice on top? That's just mean, Bioware.
Codex entries about Abominations and Revenants. They're mentioned killing the Templars sent to slay them, and the Abominations are barely even HUMAN now. They're all mutated and swollen, now merely vessels for the demons that have inhabited the body of a poor, luckless mage.
One Templar recalls looking right at an Abomination who was blasting a town apart trying to keep the Templar and his men from getting into sword's reach of said Abomination, and suddenly understanding that the no-longer-human mage wasn't luckless: the Templars had already been hunting him for using forbidden magic, and the mage realized he wasn't powerful enough to win without letting himself turn into an Abomination.
The Circle Tower, whenever you go to it, might be this as well. You see the moon behind it, and the tower is completely black. The only light is from a spot near the ground, which you assume is the main door.
The whole section where you get trapped in the Fade by a sloth demon is Nightmare Fuel. The music doesn't help, nor does the convoluted, frustrating and claustrophobic nature of the missions in the dreamscape.
The sloth demon itself, between its gravelly voice and attacking you in your nightmares, is a reasonably good Expy of Freddy Krueger. Now imagine Freddy Krueger possessing a mage.
The landscape of the Fade: a twisted, confusing mass of small islands filled with demons and spirits of the dead. And in the center, there's the Black City... always there... always in the center. The city of the Maker, the god who has turned his back on all except those who believe in him.
Golems in Dragon Age are made of equal parts lyrium and Fridge Horror. Sure, Caridin was a genius. He probably exhausted every scheme for golem-making that seemed even remotely feasible, before finally settling on that not-very-nice one. Still, he ultimately decided to stick one of his fellow dwarves in a ten-foot-tall suit of armor and pour liquid lyrium into the joints until the subject stopped screaming. That the dwarves would elevate him to Paragon after he did something like that, did it enough times to create an army... it pushes them from Deadly Decadent Court into "Good God, what the hell is wrong with you people!?" Caridin himself comes off even worse, if possible: by his own account, he failed to appreciate the full horror of his procedure until he himself became a golem. Leaving aside for the moment the moderate to severe sociopathy that such a failure implies, who made Caridin a golem? And what happened to that person, and his knowledge of golem creation? Hopefully it didn't fall into the wrong hands...
Caridin never told the court or the general public the true horror of the golem crafting technique, and it is implied by information found on the DLCs that he isn't its original creator. The dwarves were desperate and there was no shortage of volunteers in the face of the encroaching darkspawn. Then the king at the time, Valtor, decided to turn casteless, criminals, and his political enemies into golems. Caridin objected, and that's when he was turned into a golem himself.
Also alleviating it some what was the fact that all the original golems, such as Shale, were volunteers, as already mentioned. Presumably, they knew the process would be extremely painful, but were willing to undergo it to give their people a fighting chance against the darkspawn. The real horror only came when the King started throwing everyone he didn't like onto the anvil and forced Caridin to create the control rods to rob the golems of their free will. Caridin trying to oppose the King in these decisions is what got him turned into a golem himself. Until that point you could almost view the golems as the Dwarven equivalent of a Super Soldier.
A low-key example from Ortan Thaig: You've been harassed by spiders since you entered the place, and now you have to go deeper into the bowels of the earth. Oh look, more spiders in the tunnel...except, in violation of all video game monster etiquette, they're not attacking you: they're pulling back. Nothing about their behavior suggests fear. It's more like a controlled retreat where they only stick around long enough to make sure you're following them deeper - and of course, the way they're heading is the way you have to go. It's a nice and unexpectedly creepy touch.
Something about the way that Tamlen vanishes, screaming, in the Dalish Elf origin is more than shiver-worthy. Getting ambushed by what remains of him later in the game is just the icing on the cake.
In addition, in the moments leading up to this, you can say "Get away from it Tamlen...." in an attempt to warn him away. Despite the order only appearing in text, the context and the atmosphere of the scene makes the line practially ooze uneasiness despite it being completely un-voiced.
Also, think about the implications of not playing a Dalish Elf. The PC likely turned into a Shriek like Tamlen, which means there's a high chance they met their death at your hands.
The implications of not playing any of the backgrounds is pretty Nightmare Fuelish: the Human Noble is slaughtered at the hands of theri family's "oldest friend"; the City Elf might be raped (if female) and even if they still managed to kill Vaughn, prison 'won't be kind to a knife-ear who killed an arl's son; the Dwarf Commoner is executed; and the most fun of all, the Dwarf Noble is sent out to die in the Deep Roads. Only if that noble is a princess, she isn't going to die at all, is she... It's never explicitly stated that the Broodmother you fight is one of Branka's household. It could very well be the dwarven princess that Duncan wasn't there to rescue.
If you ask Harrowmont about Endrin's death, the conversation leads to the events of the aborted Dwarf Noble origin, in which Harrowmont confirms the Noble was a prince. Luckily for the prince... And the Dwarf Commoner found their way back into the carta's custody, rather than being executed. They then went on a hunger strike and died.
In the Witch Hunt DLC, you find out that the Dalish Elf was found and brought back to camp, but never recovered and ended up just dying, luckily spared the same fate as Tamlen. Dying is probably preferable to finding out they've been suffering in pain for months while you've been making good use of your second chance at life/extended death sentence.
And the mage? The mage is probably taken to Aeonar for helping Jowan, a prison run by the Chantry at the site of an old Tevinter facility. Due to experiments the magisters did there, the Fade is so thin most mage inmates get possessed by demons. If the mage turned on Jowan, he/she was probably killed or possessed and then killed in Uldred's rebellion.
Aeonar, possession, or being made Tranquil? Which one is scarier?
Abominations are pretty scary, yes, but what really plunges the player into uncontrollable terror is watching a guy get twisted into one. He screams horribly and starts glowing and floating... and then he's gone, and in his place there is a deformed, twisted monster who doesn't even remotely look human anymore. And the thing's gaze... * shudder*
Even creepier when Uldred offers to turn you into one as well.
Sten's story about the fiends of Seheron — the Tal'Vashoth — is disturbing, particularly where he describes a small farming village where he was stationed. He's somewhat vague on the details, but the implication is that a Tal'Vashoth was picking off the inhabitants one by one, leaving only bits and pieces of their bodies in the jungle for the others to find.
In Dragon Age II, you meet some Tal'Vashoth. While some of them are willing to kill humans on sight, there are also reasonable ones with good points. Sten is honorable to the ends of his fingertips, but that doesn't mean he's always reliable.
From the Golems of Amgarrak DLC comes the Harvester, a flesh golem. Imagine a gigantic mass of corpses crudely sewn together and driven to kill more people to add to it. The process behind making it was so horrible that even Branka was too disgusted to consider using it.
Amgarrak itself. Even before you breach the entryway, it's clear something's not right; the undead attack you right outside the entrance, and you see Deepstalkers fleeing in terror from something up ahead. Then you enter the fortress, and things get a whole lot worse. The Harvester scuttles ahead of you at every turn, the grisly research notes from the overseer detail its creation, and the increasingly panicked journal entries from the leader of the recovery party who went in ahead of you, as he realizes that something is in there with them, make it feel like System Shock 2 meets Dragon Age.
When you kill the Harvester and leave Amgarrak, the last shot is of dozens of the Harvester brood flooding out behind you.
The DLC The Darkspawn Chronicles examines an alternate timeline where your character died during his/her Joining and Alistair was The Hero. You play as a Darkspawn Vanguard during the final assault on Denerim. Along the way, you fight and kill members of your party including Sten, Wynne, Oghren, and Zevran. Lastly, you go up to the top of Fort Drakon to defend the Archdemon from Alistair, Leliana, Morrigan, and Dog. The end cinematic has Alistair crawling on the ground looking at the dead bodies of the other three companions. Then Alistair is impaled by the darkspawn you control. If you cared about any of your companions, this scene is both Nightmare Fuel and Tear Jerker.
Also the presence of Morrigan in that battle implies that Alistair agreed to Morrigan's Dark Ritual. Things must have been so grim that Alistair, who had to be coaxed into agreeing with the ritual in the main game, did so willingly without the influence of your character. Break the Cutie indeed...
And add to the fact that Leliana is rumoured to have been Alistair's lover in this AU. Meaning that Alistair not only probably slept with Morrigan to have a chance at a happy life with her after the Blight (against every revulsion he must have had at the thought of doing it), but one of the last things he sees is Leliana's mangled body - giving him enough time to react with horror before getting his head hacked off...
The first time you see a Joining it's pretty creepy. The guy stumbles back moaning in pain, then suddenly his eyes go completely blank (or roll up so that the pupils aren't visible, which is even worse). Then, because he's one of the many unlucky ones who don't survive the Joining, he starts choking to death right before your eyes. No wonder Jory freaked out.
Duncan's utter, glacial calm when he murders Ser Jory is this. He doesn't want to do it, he doesn't enjoy doing, but he still does it. If you had any illusions that the Grey Wardens were a purely noble, honorable, good Order of selfless heroes, they are dashed hard right here.
Even more terrifying to me was the entire concept of the Right of Conscription. So, basically a Warden can just pick you off the streets and force you to become a recruit for any reason they see fit. Result? A) Death from poison, B) death from instant murder if you try and back out upon realizing that you've been shanghaied/misled, or C) the solid gold Kewpie doll: you survive... but are still almost guaranteed a violent and premature death from fighting. I am having so many Jedi flashbacks right now, only with 90% more dying.
And even if you don't die in the line of duty, eventually you'll have to commit suicide by darkspawn horde to avoid dying a slow horrible death, or going insane and subsequently ending up as part of said horde.
Lothering fits many of the "first town" tropes, complete with helpful people surprisingly quick to join you on your deadly adventure, monsters described as tough but really aren't, and sidequests requiring absurdly low-level skills. And then you leave, and the icon turns into a skull and crossbones. The Blight hits Lothering, and literally wipes it off the map. Excepting recruited party members and a father-and-son merchant duo, you never see any of the people there ever again, despite having lengthy conversations with a dozen or so of them. Count the NPCs on your next playthrough, and remember that excepting one family (if you're nice to them) and one child (if you talk with him), you never saw anyone leave. (Another exception is made in the second game—the Hawke family lived in Lothering, and most of them escape, but they're full-on hero types.)
Hawke does get letters from other survivors of Lothering. Some the Warden met, some s/he didn't.
Just the Deep Roads are freaky enough.
One issue with the end of Witch Hunt, if the romanced male Warden goes through the Eluvian with Morrigan, he is effectively abandoning Finn and Ariane in the Mother's former lair. Remember that you just defeated a giant spider outside not 5 minutes ago; what's to say nothing else is waiting for them outside? Also, is Ariane likely going to escort Finn back to the Tower where Templars could perhaps hold her for information on the location of her clan and their Keeper? Taking that into consideration, if they parted ways and Finn is left to return on his own, what's going to happen if a Templar patrol comes across him and believes him to be an escaped apostate?
That is not so terrible. The worst monsters were cleared from the surface when they entered and, judging by the fact that we skip from the entrance to the Mother's Lair, there was nothing worth noting in the tunnels. Finn does not need Ariane to escort him the whole way to the tower; despite his age he's (surprisingly) a high-level mage. If he encounters Templars he can tell them exactly what he's told you - he has permission to leave the tower to perform research. If they don't believe him, then they'll take him back to the tower - exactly where he's going anyway.
It also stands to reason that as Warden-Commander of Ferelden, the Warden could easily draw up documents that politely tell any authority who might hassle them to go swim across Lake Calenhad in full armour. They certainly had enough time whilst travelling across half of Ferelden.
Then too, Knight-Commander Greagoir was very willing to let the mages he guarded go wherever they wanted as long as they sought permission through proper channels. It's highly probable that Finn (and any other mage who left the tower under authorized circumstances) was already carrying documentation with the Knight-Commander's seal to show to any Templars he might encounter.
Before leaving the tower, Finn mentions that he had authorized permission to leave the tower but never had the oportunity to do it.
The Pride demon from the Mage's Harrowing. True, you never face it in-game (at least, not in the first one), but it gets one HELL of an scene. Mouse drops all pretense of helping you, and begins his transformation; Suddenly, the camera angle changes, and you're looking down at your character over the shoulder of a HUGE demon (most of whom you can't see); And then he implies that this isn't over.
No mention of the Warden's Keep DLC? The Warden is brought to Soldier's Peak, the former Warden HQ, by the great-great-grandson of the former Warden-Commander, Sophia Dryden. Upon entering the gates, the Warden and their companions witness the scene of the final battle at Soldier's Peak. Following the cutscene, the Warden is attacked by the corpses of the former King Arland's men as well as those of former Wardens. But perhaps the greatest shock comes on floor two. When the party enters a certain room, someone calls out to them. It turns out to be the corpse of Sophia Dryden, which is being possessed by an escaped Fade demon. Worse, she is visibly rotting around the eyes and even comes with a very unsettling voice.
While conducting the "Rescue the Queen" quest, you can hear ambient dialogue between some of Howe's soldiers, who are talking about how much they hated being sent to Castle Cousland in Highever. They describe the corpses littering the hallways, the unsettling quiet of the place and the sense that, given half the chance, the villagers would happily kill them all in a heartbeat for what they did on Howe's orders. That's bad enough if your Warden comes from any of the other five origins; it's not a pretty picture to have in your mind. But if you're playing the Human Noble, remember that your Warden knows all those dead people (and is related to a few). Imagine overhearing a conversation like that about your childhood home and the people who lived there with you.
Giant Spiders are pretty prevalent in the game, creeping in from overhead or behind, which is Paranoia Fuel, however what seals them as this is their overwhelm ability. As stated they overpower your character, pin them to the ground and bite them to pieces. The dark red blood that flows out is off the scale.