Half-Life (series)/Nightmare Fuel

Describe Nightmare Fuel/Half Life here.

But what part of Half Life DOESN'T qualify?

Valve software's attempt at an FPS narrative, namely replacing the generic A Space Marine Is You (battling for the fate of the universe, natch) with a Badass Bookworm who seemed to be stuck in the middle of a bad situation, made players much more sensitive to the atmosphere than before. And with each release, Valve has been making atmosphere an increasingly bigger part of the experience.

Series-wide

 * Of course, all headcrabs count as a form of unleaded Paranoia Fuel, due to their tendency to show up absolutely anywhere. Hiding in alleyways, skulking in storerooms, lurking in air vents, ...
 * This is prevalent enough that it's actually Lampshaded in Half-Life 2's second chapter; when Barney turns the lights on in the HEV Suit room, he's immediately ambushed by Lamarr, Dr. Kleiner's pet headcrab.
 * Barnacles... You suddenly find yourself being lifted into the air and looking up to see that red, bloody maw. Just seeing their "tongues" hanging around is Paranoia Fuel.
 * One of the things that Valve is well aware of and comments on in their dev commentaries and interviews is "Gamers don't look up." One had to wonder how many Valve fans have been broken of that particular quirk.
 * There's the scene in Half-Life 1 where the silhouette of a scientist in a darkened room gets dragged up by the neck and then gibbed with a wet crunching noise. You will not hear the sound of a dog panting the same way since.
 * G-Man. There is just something unsettling about him... Basically, he looks and sounds like a creature doing an only-adequate job at pretending to be human.
 * It's the pauses. "Rise and... shine, Mr. Freeman. Rise... and-shine." *shiver*

Half-Life

 * Don't forget about the Ichthyosaurs from the first game. These underwater dinosaurs would swim at rapid speeds towards you growling. Not to mention that, in a particularly annoying bit of realism, most of your best guns don't work.
 * The underwater sections of Half-Life; low visibility, the potential yield of the average Xen monstrosity lurking in it...
 * Nihilanth. The fact that he's basically a humongous floating fetus with a head four times the size of his body, plus his scream of "FREEEEEEEEEEEMAAAAAAAAAAAN!" when you finally reach him...
 * Its level design isn't looked on kindly, but, good lord, Xen. Once you're through that teleporter, there's no going home - and you're in an incredibly hostile alien world that's a prime source of Nightmare Fuel - it consists of small islands suspended in a great void of complete emptiness. The Alien Sky doesn't help. And inside, it became even more freaky, with Alien Geometries. The level design may not have been great for gameplay, but it does have atmosphere.
 * The trees. Good God, the TREES.
 * Blast Pit. You hear the banging noise before you get to see what's causing it, and that echoing metallic sound is with you throughout the whole level. And how about those moans? Or the "death sound" that comes after you successfully ignite the rocket engine.
 * The section with all the conveyor belts and large vats of questionable substances. Not very scary... until you notice that there are other things besides you being flung onto the conveyor belts. They're body parts.
 * The Gargantua is pretty damn scary. It's huge. It has a giant red eye. And it will kill you on first contact. While the terrible graphics (compared to Source) make it less horrifying, its sheer presence makes the player want to rethink their path. Not to mention when they're chasing you. Who here didn't look back when in the garage in Surface Tension, when they knew there was a huge-ass monster chasing them?

Opposing Force

 * Adrian Shepherd's eventual fate. Trapped on an aircraft floating through some empty other dimension - for all we know, for eternity, as this fate is designed to preserve him. ...AndIMustScream indeed. One can only hope he's in stasis and not cognizant.
 * The "We Are Pulling Out" chapter. The chapter starts after you jump onto a Black Mesa tram after the Resonance Cascade has already happened. You then proceed to ride the tram backwards to the surface, while the all-too-familiar automated voice relays it's horribly chopped and distorted message (also, there's Vortigaunts on the way up that will attack you if you don't shoot them). A short time later, after spending a few minutes with an incredibly socially awkward security guard, you reach the helicopter on which you and your fellow soldiers will leave. As you head towards the helicopter, The G-Man That bit just really gives you the chills, and it just makes you wonder what the G-Man's real intentions are.
 * Believe it or not, G-Man does seem to have good intentions. If you stick around the area after the chopper flies off for a half a minute, you'll hear a pop and see a bright flash. Guess? The chopper that took off just now, the one that Shepard could've very well been on if the G-Man hadn't intervened, exploded no less than 30 seconds after it took off.

Half-Life 2

 * Stalkers are mutilated, forcefully cyborged vestiges of human beings, who are completely dependent on their mechanical limbs, and forced into service to alien rulers. Alyx prays that the stalkers don't remember who they were, and they are also implied to be immortal. They'll continue to live as long as they're refueled and properly maintained. Knowing how brutally efficient the Combine are, that's almost a guarantee. Most of the stalkers' internal organs have been removed, and they are dependent on some kind of saline solution provided by the Combine for survival. Other than that, however, they can work in just about any hazardous environment.
 * By the way, the original model for Stalkers was much, much creepier--this can be found in the mod Missing Information. It's basically a skeleton with a mask and limb replacements. Stalkers are meant to evoke the Uncanny Valley effect; their appearance is meant to tell the player, "these things were human once; whatever they are now wishes they still were." Pray that you are never close enough to them to hear these sounds.
 * Stalkers only get creepier once you realize that basically every "Synth" creature (Striders, Hunters, Gunships, etc.) the Combine has thrown at you thus far is approximately the same thing, except made from other extradimensional species and heavily weaponized. The Combine have been doing this since before they neither knew nor cared we existed, and likely think little, if anything of it at all. Oh, and for those of you who haven't realized it yet, the Combine soldiers that you fight throughout the entire game are the Combine's newest synths. Even the civil protection, who are mostly human, and, if they want to get a higher rank, they have to be brainwashed and castrated in the same way as the soldiers.
 * The long-legged "fast" headcrabs, which produce skeletal, inhumanly dexterous zombies that slobber like dogs while they mauled you. (Not to mention they all but pull a Personal Space Invader in the process of mauling the player.) And the way they scream when they are jumping at your face from the building across the street. Oh, and there's the little fact that they are missing all of their skin, along with most of their organs and quite a bit of muscle, the head nothing but a skull. And then, near the end of "Anticitizen One", the game starts throwing Fast Zombies at you that are on fire.''
 * And even before you come into their line of vision, Fast Zombies will hunt you down no matter how large the distance between the two of you is. They're essentially organic zombified T-600s.
 * After Grigori gives Freeman a shotgun, he is immediately ambushed by half a dozen Fast Zombies, partially to inform the player that yes, they can leap at you from across buildings, partially to make them crap their pants. Soon after that the player must then wait for one of the series' Ridiculously Slow Elevators, at which point several more of them attack. If you're smart and close the door to the room, realising they can't open it, the proceed to crash through the skylight.
 * The introductory vision of the Fast Zombies may be one of the scariest moments in Half-Life, ever: the salivating dog sound comes, followed by approaching thumps of the zombies' leaps and bounds. You look around and then you see the shadow of the Fast Zombies leaping across buildings with the full moon behind them. You just KNOW they are coming for you.
 * The encounter waiting for Grigori's cart ride to the church where Fast Zombies keep arriving and climbing up to your roof. Just hearing the sound of creaking drainpipes is enough to make your skin crawl.  
 * Not to mention the original zombies now screaming and moaning in their human voice... along with the ability of the damn headcrabs to jump off of killed zombies... ugh. Oh, and when you set them on fire? Shudder.
 * If you listen to the "classic" zombies, their babble gives the very disturbing implication that the victim is still sufficiently aware of his predicament to scream things that sound like "Dear God help me!" and "Why why why?" The headcrabs not only excecute Grand Theft Me, but the leave enough of you intact for you to know what it's doing to you!
 * Headcrab zombies on fire make a disturbing "human dying in agony" sound, especially chilling if you are the one who set the zombie on fire initiating the torture and death of the underlying innocent human. Apparently, for headcrab zombies, the human consciousness is gone but the part that feels pain remains. In fact, if you play a headcrab zombie's cries backwards, you can hear . A sample is here. With that in mind, their death grunts almost sound like sighs of relief. Eugh.
 * The even scarier venomous black headcrabs, which reduce your health to 1 whenever they bite you, hiss like rattlesnakes, attach themselves in groups of four to a corpse, which attacks by hurling them at you and cannot even be killed by a single hand grenade.
 * The developer commentary points out that black headcrabs are specifically their favorite ambushers, because though they were terrifying (to the point that play testers would waste ridiculous amounts of ammo in a manic attempt to kill them), they were physically incapable of killing the player. Understandable, since getting smacked by one and then having fast zombies bearing down on your ass is NOT a fun experience.
 * Black headcrab zombies have a more subtly creepy noise: If you have the sound high, or headphones in, and somehow manage to get close to one of those things without it detecting you, you'll discover that it makes a soft, wet, labored breathing sound when idle. Slightly off-putting at first, until you realize that, in spite of being a poison-bloated corpse covered in oversized rattlesnake-parasite-things, with all of the flesh over its spine stripped off and presumably eaten, there is still something alive under there that has to breathe. Also, poison zombies laugh and moan when they die, presumably upon realizing their suffering is over.
 * The poison headcrabs get worse in the modification Smod if you have Call of Duty style health enabled. Getting hit by a poison headcrab creates an absolute Interface Screw, leaving you even more vulnerable to any other enemies around.
 * Often times, a black headcrab hits you and you are flung at high speed. Off a high rooftop or platform. *Crack!*
 * When you finally escape Ravenholm, exiting the tunnel, breathing that fresh, City 17 air, recovering that ammo from the crate - OHSHITPOISONHEADCRABDIEDIEDIE
 * Father Grigori may be helpful and overall act awesome. But the chapter ends on a bittersweet note, didn't it? You get out barely alive, but are left to see Father Grigori fend off massive hordes of zombies in a fight that he'll ultimately lose against and be mauled to death--and join their mindless, screaming ranks. Then when you got out, you took a moment to yell a bit and take in some fresh air and DAYLIGHT. Just a moment... before some damned hellhole creature hurtles at you and kicks your health down to 1. Oh, and then there are the snipers...
 * If you stay around for a little while, to see what happens to Father Grigori, you will continue to see a wall of flames, and Headcrab Zombies struggling to it, while shotgun bullets fly through the fire and headshot the Headcrab Zombies who drop dead one by one. You expect the shooting to go on forever. It doesn't.
 * The mine, filled with an unholy amount of infinitely respawning headcrabs, with that horrible chittering the poison ones make and watching the fast ones sprint about on their vile gangly limbs. The solution? A Han Solo-esque sprint 'n' scream across the bridges and into the pool of water.
 * As the player is trying to reach Barney (who is pinned down by snipers), the path goes down to the basement. It is flooded to knee-level. A Resistance soldier asks for help. The wounded soldier, when the player gets there, says, "We came down here looking for shelter. Little did we know the place was infested!" With sounds of splashes and moans coming from all around you, you suddenly become aware of the fact that you are surrounded by Zombies.
 * The oceans are filled with millions of writhing carnivorous lampreys which swarm around you screeching and stripping your flesh off.
 * Half-Life 2 will bring back lovely memories for veterans - during the teleportation accident, Gordon is plunged into a random pool somewhere. An Ichthyosaur comes charging at him from the gloom, and opens it's mouth, about to eat him, when he teleports away again. Its original role was to keep the player out of the water, but that was replaced by those leeches.
 * The Combine Advisors, giant alien slugs equipped with some bionic attachments. If that wasn't creepy enough, they also have Psychic Powers, which are used as an Interface Screw in Episode One and Episode Two.
 * Manhacks. Just the noise.
 * Even the sound aside, the very concept of Manhacks is chilling. Flying circular saws that come at you in swarms, spattered with blood from previous victims?
 * Original design documents called for the "Manhack Arcade" location. In other words, some of those Manhacks would be controlled by arcade machines being played by your fellow humans, who would have no idea they were actually killing people.
 * The zombies have nothing on Antlions. Guards in particular.
 * Via quirk of the Source engine, Antlion Guards can actually fling Striders with their ramming attack. Whether this is hilarious or even more disturbing is up for debate.
 * Then there's the part in Episode Two when you have to go into a Guardian's nest and run around its tunnels, with the Guardian possibly right behind you.
 * Throughout Half-Life 2 the player finds several rotting bodies in various places. Not just dead people, but actually rotting corpses. Their flesh is a pallid blue, their clothes are degrading, and worst yet, their face looks like it's been burnt to a crisp. The eyeballs are gone, their hair has been torn off, and their teeth are chipped and barred. Think that's the worst? Many time you will also find bodies that have been completely immolated, leaving nothing but a charred crisp with burnt fat at the edges. The mod Minerva: Metastasis even has a gas-chamber in one of its levels with the above-mentioned bodies strewn about with blood liberally splattered on the walls.
 * One particularly infested area in Ravenholm has a small, easy-to-miss passage containing a corpse and a pistol. Even worse, there's a mattress, implying the poor soul had been living there for quite some time, hearing everything.
 * The zombie corpses are also disturbing, with their claw-like hands, their bent-back heads, and their mouths wide open in a frozen scream.
 * One of the Striders' attacks involve impaling an enemy with one of their legs and later shaking it off like something off a person's shoe.
 * The G-Man sighting in Half-Life 2's level "Anticitizen One", in which he appears on an unplugged television standing in a rowboat with a crow on his shoulder. It would be goofy if the music accompanying it didn't make it horrifying.
 * The sewer with the burnt corpses and the creepy music.
 * The toxic tunnels under City Seventeen. You have to cross the radioactive waste by placing tires, crates etc. with your gravity gun and hopping from one flimsy platform to the next. This is nerve racking enough, without the fact that the zombies are apparently immune to radioactive waste. The first time you jump off down onto a tire, only to have a headcrab zombie rise up two feet away... terrifying enough that you normally leap backwards into the fatal green sludge. Even on the 3rd attempt the sudden moans of zombies behind you often causes a wild spraying in all directions with whatever gun's in hand.
 * Those moving walls. Their gigantic, foreboding appearance, their tendency to begin moving with no warning except an unearthly groan (that sounds suspiciously like the Ravenholm motif), ready to crush you underneath them. Not to mention the implications that wherever the Combine place them, they will steadily move outwards, destroying everything in their wake until they transform the entire region into barren wastelands...

Lost Coast

 * According to the developer's commentary, the developers purposefully designed certain areas of the stages to be distinctive "arenas" by making them large, open areas. They believed this would make the player feel exposed. It does.

Episode One

 * The Stalker Car sequence. Nothing quite like waking up to see a thrashing half-human face screaming and gurgling at you. And of course poor Alyx was pinned to the wall by one...
 * Episode One introduced the Zombine, a headcrabbed Combine soldier, a tougher, slightly slower version of the standard zombie (although it did sprint occasionally) with zombie flesh bulging out from the seams of its combat armor. While its inclination towards pulling out a grenade and charging the player waving it above its head is unsettling, it is the deeper implications that really freak you out. Unlike other zombies, shooting the headcrab off does not reveal the grizzled, bloodied face of the human victim but instead only the soldier's lower jaw fused to the top of the spine. Despite having no remaining brain or even head beneath, the Zombine moans and cries have a distinct synthesizer edge to them. Listen closely and you can make out the disrupted, mutilated calls of the former solder. Groaning a muffled 'medic' when shot, informing allies of 'biotic' (Combine jargon for antlions) when milling around, and gurgling out 'grenade!' when they start their suicide run. All horrible, but consider that the transhuman nature of the soldiers means the Zombine likely automatically broadcasts these cries to any still non-zombified soldiers in the area. If the Overwatch soldiers are still human enough to still feel terror, one can imagine the terror of hearing the squad conversation over his comm channel slowly being replaced by a Zombine cacophony as the soldiers fleeing City 17 fall one by one to the headcrabs...
 * Listen to their audio clips here. It's made all the more creepy if you consider that it's either the headcrabs tying to talk through the body but can only repeat the last words said by the deceased, or the soldier is still alive and trying to contact his squadmates.
 * "In-fest-a...*groan* -tion..." "perimeter is...*shuddering breath* not secure" "Necro-tics... inbound" "they're inside" "over...run" "help...me"
 * The City 17 Underground in the "Lowlife" chapter of Episode One. Like Ravenholm before it, it serves absolutely no purpose other than having the player in tears, and it works. The majority of the level is in complete darkness (as in you literally can't see your hand in front of your face) without using Gordon's Ten Second Flashlight, and the place is swarming with Headcrab Zombies, which will actually spawn faster if the flashlight isn't activated. As a small relief only a few zombies are encountered at any one time... until the last part that is, in which the player must wait for another of the series' notoriously slow elevators while an unending horde of zombies crawl out of the darkness. There are few things worse than a Fast Headcrab Zombie which was completely invisible a second ago leaping at you from nowhere and clawing at your face while a nearby Zombine decides to become a suicide bomber.
 * "Alyx, normally I love working with you, but goddamnit! Stop making zombie noises behind me when we're heading into headcrab territory!"

Episode Two

 * You witness the Combine Advisors telekinetically hold someone in the air while sucking the guy's brains out. Oh, and just to cap off the aforementioned sequence? It twists the rebel's corpse in half, and tosses it aside like an empty soda can.
 * Ugh. Not to mention that the appendage that they use to do the brain sucking is remarkably disturbing itself, they It's then that you realize just how deep into this mess Gordon has gotten himself.
 * In Episode Two near the end of the mine, there's a rotting corpse sitting in an armchair. With a shotgun lying next to him and a large splatter on the wall behind him. It's easy to miss with all the weapons and corpses lying around and blood splatters being part of the scenery.
 * Valve seem to have a game of oneupmanship going on in Half-Life 2 and the subsequent episodes, whereby they try to top whatever terrifying enemy they pitted you against last with something even worse. The Zombine was bad but, ladies and gentlemen... the Hunter. The manner in which it stalks you at the beginning of Episode Two - at one point you flick a switch, you turn around and it's there, staring in at you through the window before calmly shuffling off. Creepy, even before . And then the game lets you forget all about it before about 5 of them attack you at once. They are horrible in a way the much bigger Striders aren't; they flush you out with flechettes, they get in your face and remind you how puny you are in comparison to the Combine's creations, with their gait and their horrible grating shrieks.
 * They're kind of like the velociraptors in Jurassic Park. Compact, quick as lightning, and absolutely deadly. Making the comparison even worse are the parts in the game where you are ambushed by them, or moments where it is as simple as hearing them in the distance, mixed in with the background noise. You might not be able to see them, but they're around, somewhere...
 * The second (third?) time Hunters are encountered has them as support for a Combine ambush of Gordon and Alyx. As if what appears to be an entire platoon of Combine soldiers trapping you in a confined space and sending waves in after you while their comrades constantly fire at you from outside isn't bad enough, Hunters soon crash into said confined space and do everything in their power to turn you into Ludicrous Gibs (to quote Alyx: "Three at a time; it's too much!"). The next time they're encountered is after the game is "kind" enough to give you a false sense of security, having them storm into the previously-safe White Forest and re-introduce them brutally killing a pair of rebels right in front of you, and the area in which they're fought is specifically designed to ensure that no matter where the player goes, the Hunters can always find them.
 * You are crawling under the gunfire of the Combine's wall mounted autogun when all of a sudden you hear the hideous screaming of the Fast Zombie. It is then you realize that these fast moving monstrosities don't need legs to keep up with you. Not too far from there you see one of these legless things get shot up by a Combine Soldier following slowly behind it. This happens a mere few feet away as its skinless corpse lies motionless. (And according to the commentary, this sequence was added as comic relief.) And you hear zombies on fire everywhere.
 * You are trying to get the car across the bridge, you encounter a dumpster that shakes and growls like a Fast Zombie. You throw in a grenade. Problem solved, right? He throws it back.
 * And in that same spot, at the beginning. Oh God. That poison zombie hacking and wheezing, and it's still a shock when you reach it. Not to mention that one hallway that has a couple zombies in it, not bad....
 * You're in the car with Alyx being chased by the Combine. Amidst the chaos around you, you spot the form of a Fast Zombie running across an overpass. Odd. You speed toward a ramp and as you make the jump he makes his... onto your car. It blocks your view of the road. The headcrab starts to come off revealing its skeletal face. Alyx kicks the zombie off and you bank a hard right to avoid hitting a wall. You wish that Alyx had kicked the Damn thing off sooner...
 * That farmhouse on the road in "Under the Radar". Early in the chapter, you see smoke off in the distance. Getting closer, you see it's a crashed Advisor pod, mangled almost beyond recognition. Nearby, there's a farmhouse and a barn. When you get closer, all of a sudden the screen distorts weirdly. It's like the encounter in the Citadel in Episode One, but less hostile, more probing. You expect an Advisor to come cresting a hill or out of the hole in the roof of the barn, but nothing happens. You have to look for it yourself, and it's terrifying. Nothing Is Scarier, indeed.
 * Then you actually find the damn thing. The area itself is dimly lit and decaying, giving off a creepy atmosphere in and of itself. The body of the dead rebel doesn't help either. Alyx finds the life support energy ball, and you knock it away with the impression that you'll kill the Advisor in its sleep. Wrong. An alarm sounds, and you and Alyx are grabbed and suspended in mid air as the pod opens up. There's the Advisor, awake, alive, and pissed.
 * The G-Man