Halo (series)/Nightmare Fuel

"Gravemind: Now the gate has been unlatched, headstones pushed aside...corpses shift and offer room...a fate you must abide!."
 * Everything Flood-related. Especially the oozing infected pustules which spewed enemies when shot, and getting to see living allies and enemies mutate into Flood on screen.
 * Not to mention that if you manage to shoot an infected soldier in a way that doesn't make it explode, and walk up to its body, you can see one clinging to the spine...
 * Perhaps the creepiest parts in Halo 3 are when the Flood start talking to you. Hearing the Gravemind cursing and taunting you as his minions swarm you from every direction can chilling.
 * Bungie recently had an article that talked about the creative origin of the Flood. Upon being shown a single still picture of just one Flood form, the ESRB demanded the game be given an M-Rating. And that's without the lovely things mentioned above.
 * The psychotic Marine in Halo: Combat Evolved, as well as the insane one considering suicide in Halo 3 add to the horrific atmosphere.
 * Same voice actor for both. That guy is disturbing in-character.
 * The first encounter with the Flood in the first Halo. The downed Pelican ("We're under attack by an unknown enemy! It isn't Covenant!"). The abandoned artillery pieces, all facing away from you. The grunts that used to flee at the mere sight of you run right into your kill zone in a panicked retreat from something even scarier, and that "something scarier" seems to follow you, as you hear strange noises and see strange shadows on the trees, who appear as "allies" on the HUD...which disappear one-two seconds later without any trace. You follow the trail of carnage into a vast, underground complex, all painted in Covenant blood and deserted save for one man; a wounded Marine so insane with terror that he can't tell friend from foe. You wander deeper and deeper into the ancient ruin, and finally find what's left of the rest of his unit. You download the feed from one unfortunate soldier's helmet camera, watch the entire squad get slaughtered... and then your radar goes wild.
 * And the thing is, is that their introduction was a total surprise. There had been no signs in the instruction manual, or anything in the game that indicated that it was going to turn into a horror movie. Yes the grunts were running away, but that could have been from anything. Everyone remembers when they first walked into the last room, and suddenly a bunch of red dots appear on their radar, and then these jellyfish appear out of nowhere and start jumping at the screen.
 * One line that the Halo 3 a marine said gives a good description of an infected person: "I could see it... WRITHING UNDER THEIR SKIN!"
 * Fun fact: according Halopedia, the Flood are extragalactic. They've already destroyed at least one galaxy.
 * That's actually a bit in question with the revelations of the newest Halo -book (canonical status unconfirmede) - but it's no less nightmare fuel, as the Flood in this book is apparently a misplaced revenge from Ancient Aliens - precursors to even the Forerunner, possibly precursors to even the actual Precursor race of the series.
 * The Forerunner Trilogy is canon, and elements of it will be incorporated in Halo 4. The second book of the trilogy, which just came out recently, revealed that the above was in fact, a deliberate deception on the part of the Flood. The "last" Precursor is in fact '' . While there may be a link between the Precursors and the Flood, the second book a goes a long way in resolving the discrepancies between the Forerunner Trilogy and the video game trilogy.
 * Adding to the creepiness of Halo 3, nearly all of Cortana's transmissions are either outright skin-crawling in their delivery, if not outright terrifying, with some carrying threatening tones in her voice.
 * What's worse is that in "Cortana," as the level progresses, Cortana's transmissions become more and more frantic and confusing, eventually becoming incoherant screams of pain... before she suddenly starts speaking in a lifeless monotone, rattling off her serial number. Not only that, at that point, her graphics turn from blue to green.
 * "It was THE COIN'S FAULT! I wanted to KEEP YOU SAFE! Make you STRONG!" Made a thousand times creepier if you've read The Fall of Reach and know the significance of that transmission.
 * A collection of lies, that's all I am! Stolen thoughts and memories!
 * The Flood music themes. Features Ominous Ethereal Choir, Psycho Strings, and Scare Chords. Especially the one during the level "The Oracle" that has distorted reversed voices (during the battle between the Heretics and the Flood in the lab).
 * Browsing the terminals in Halo 3; between the lack of noise in many areas you find them in, the odd "swooping" sounds as you interact with them, the random alarm prompts and the oddly incoherent messages based around the apocalypse of the Forerunners and talking about the death of millions in many cases, it all adds together into a Nothing Is Scarier form of creepiness.
 * "I SEE YOU RECLAIMER". Even more creepier than you realize that the one who sent those words was.
 * "I am a monument to all your sins." Plus, there's no telling where those awful tentacles will pop up next...
 * Captain Keyes' fate, getting turned into an immobile mass of flesh, only to get the sweet release of death when Master Chief burns him.
 * According to the second book, he was slowly losing all his memories as the parasite took over. Torwards the end he was struggling to remember his own name! Furthermore, if you'd been around five minutes quicker, you'd be able to save him.
 * In Halo: Anniversary, under him there's a terminal containing his last moments from his point of view. He's repeating his serial number, and they're rifling though his memories...We see Miranda, and Dr. Halsey, and those memories are being stolen, and he knows it, and his voice keeps switching between normal and Flood-toned. And then at the end, he swears "You will not have me." And then the Flood speak again, and now it's the Gravemind: "We already do."
 * Possibly Gravemind's creepiest line in any game. After Truth dies and Gravemind betrays you, massive tentacles burst up from beneath the platform the Chief and the Arbiter are on. This is then followed by insane, diseased laughter, followed by this:

""Infected individuals combined their resources to force other humans to become infected- usually by cannibalism of a sacrificial individual, induced to grow to prodigious size before being consumed while still alive.""
 * This is not your grave...*incomprehensible gurgle*...but you are welcome in it.
 * Or the Grunts. Yes, you heard right. They look really vicious without their mask, and we've all had at least one time where a Grunt saw us without knowing it and shrieked, causing us to jump. Fiends.
 * Or when the Elites (especially camouflaged Zealots) roar, or when they're right next to you and you bump into them (in Halo 2 they give a shriek of surprise as well!). To be fair, all the aliens look pretty frightening in some way, whether they actually look scary like the Jackals, or are just scary to fight against, like the Hunters. It's made even worse in the first game by the fact that half the time you're in dim corridors and it's incredibly quiet, because of the suspense.
 * The Hunters have no actual limbs. They're comprised of a bunch of worm-like things (1.4 meters) called Lekgolo that come together under their armor. They also make up the Scarabs.
 * You haven't seen a Hunter in action until you've seen a pair use those damn Assault Cannons (essentially, semi-auto high-explosive rocket launchers with Bottomless Magazines) to wipe out more than 15 friendly Marines in less than 10 seconds during "Tip of the Spear" in Halo: Reach . You duck behind a rock, reload your assault rifle, and when you come back into action, every last Marine which just got dropped from that Pelican is gone in a horrible subversion of The Cavalry.
 * When you kill a Brute, you may see his eyes twitch.
 * Halo Reach is introducing a new piece of wildlife called the Gueta that could give the Flood a run for its money. For those not in the know, think a Deathclaw but with elongated features, and are actually capable of killing entire squads of Elites, which are still feared at this point. The kicker? You encounter these things at night.
 * The level Cortana in Halo 3. There's a reason that's the least-played level!
 * "We exist together now. Two corpses in one grave".
 * For those who don't know, this level is the one where you essentially break into the Flood's house in order to rescue Cortana. The "house" was originally a spaceship, but almost every wall, floor, and ceiling is covered in "Flood matter." Its kind of like the halls of an insect hive. The Flood will attack from all sides (including up), as you try to navigate this somewhat-nightmarish landscape. It doesn't help that the level is somewhat confusing; many players get turned around easily even after several playthroughs. As you fight through it, you get to listen to Cortana's tortured, disjointed rantings and Gravemind's apocalyptic proclamations.
 * As you progress through that level, the Cortana moments get increasingly crazy, and horrfic, while the Gravemind moments get more and more "insane evil genocide monster."
 * "This is UNSC AI Serial Number CTN 0452-9. I am a monument to all your sins".
 * Pvt. Jenkins, who got overran by the Flood and survived! Acording to the second book he ripped his arm off to try and kill himself.
 * having a complete mental breakdown at the last level of Halo 3 and goes through a rampancy. It's very scary to see such an affable, if slighty annoying, character completly turning 180 and becoming an extremly dangerous foe who has lost his mind. Before it, you don't expect him to be a harmful threat but then he proves you wrong big time by and burning of John 117 (read:you). When you fight him, nothing works. Grenades, pistol, machine-guns, sniper rifles, plasma guns, anti-tank weapons - none of them work. The only thing you can do is make him snap even further, and pretty soon you're start feeling your heart pumping from desperation alone. The red eye works too for the fear factor.
 * The encounter with Hunters in The Silent Cartographer. It starts with being dropped on to the beach by a pelican, surrounded by allies, as you storm through countless easy covenant enemies and enjoy the pretty scenery around you, you even get a warthog halfway through, making the level even easier than it already is. But wait, the trees up ahead look too dense to drive through, so you begrudginly leave your allies behind and head forward, figuring that if it was easy so far, what's the worse that could be up ahead? Suddenly you come upon a clearing, and in it are two, huge, lumbering, armor plated aliens making loud groaning and roaring sounds in the distance. Though intimidated slightly, you step forward, figuring that they probably can't be as tough as they look, only to have them shoot a series of explosive green blasts at you. You try to close the distance a bit and hope that melee will be a bit more effective, only to fly several feet forward, after realizing that one of them has hit you from behind. The next one then charges at you, like a roaring, alien freight train, and swings it's massive arm at you, killing you in an instant.
 * In Truth and Reconciliation, two of them drop down from the grav lift while ominous music plays. At night. It's arguably worse.
 * To make it worse, the encounter on Truth and Reconciliation makes them seem unstoppable if it's your first time playing through a Halo game. Anyone who played Halo: CE as their first Halo game knows the sheer terror that pair brought upon you, cutting through all of your troops like their were nothing, deflecting sniper rifle bullets straight off of them, and grenades only making them flinch.
 * The hidden messages in the second and third game. In Halo 2's soundtrack, we have Mausoleum Suite, a song that plays during the intro cut-scene of the The Arbiter and during the confrontation with Tartarus in The Great Journey. In this track, a voice can be heard in the background. At first, it seems intelligible, but play the song in reverse and you get this: http://www.youtube.com/watch?v=C Bd A Ay X Em Jg. Fans like to speculate that this is Mendicant Bias. In Halo 3, we have Dread Intrusion, played during the level Flood Gate. A voice similar to that of Gravemind's is heard. Play it in reverse and you get this: http://www.youtube.com/watch?v=Ysp Mc Ff Cm ZA. In the level The Covenant, when the flood shows up, the song Black Tower is played. Again, a voice is heard. Play it backwards, and be treated to this: http://www.youtube.com/watch?v=RH 39 l 43 p W Gw. This appears to be a reference to T.S Elliot's poem The Hollow Men.
 * The Flood in the Forerunner novels. Apparently, before they were the twisted space zombies we all know and love, it was simply a dust which genetically altered certain animals and spread benignly via concumption. Eventually, it spread to humans, subtly altering their behaviour, until, when it became a widely known problem, it didn't bother concealing its nature. To quote a description of how it was spread:


 * What it did to some Forerunners was closer to the version seen in the games, thought more horrific. One variation was basically dozens of Forerunners mashed together into a huge snake... centipede-beast... thing, complete with scales grown over their skin, which moved using dozens of limbs stuck in its side. And the Forerunner (at least, the one whose head was melted into its chest) was fully conscious.