Nightmare Fuel / Marathon
Seven hundred and sixty one armless and legless corpses float around the inside of hangar ninety six, and they are all screaming at me.

I once boasted to be able to count the atoms in a cloud, to understand them all, predict them, and so did I predict you, but this new chaos is entirely terrible, mindless, obeying rules that I don't comprehend. And it is hungry.
-Durandal, Ne Cede Malis

Even back then Bungie knew how to scare the shit out of us.

Examples of Nightmare Fuel in the original games:

  • The dark and claustrophobic design of the Colony Ship Marathon gives plenty opportunities for this, the first of which is meeting the S'pht compiler face-to-face in the very tight maintenance tunnel. It's occasionally hard to tell whether the dark rooms, claustrophobic hallways, and stealthy enemies are meant to be scary, but boy howdy, they are.
    • If you're not suspecting it, the first encounter with an alien- not even ten seconds into the game- is to have it make a loud, startling screeching noise and run after you. It is a VERY effective jumpscare, even if it isn't too much of a threat.
    • The Jjaro installation from Infinity is one big call back to the first game, now with the addition of ambient sounds that are described in the game's editors as the ship creaking, but can be easily interpreted as the voice of the sleeping god struggling in its prison.
  • In the first Marathon game, when you blow up a BOB or another player, they explode into a bunch of intestines, gore, and brown mush.
    • And in the next two games, they explode into flesh-covered bones in a pool of blood.
    • Also most of the aliens, particularly the Enforcers, who, in the second and third game, pop.
      • And other aliens native to the planet fly towards you and EXPLODE on you. Not pretty.
  • In the first game, you can see BOBs trapped in Pfhor stasis pods on the alien ship. Their faces are pale and the pods are pulsating.
    • Other things in the alien ship pulsate too. It seems as if most of the ship is organic.
  • The Pfhor seem to be making artificial humans, which are called simulacrums. They run up to you and explode, and it's almost impossible to tell which one it is. The creepy thing? You're told that they're robots, but sometimes, it's implied that they're captured humans ("Innocent Colonists") who had their blood replaced with a yellow liquid (that's explosive, too) and they had their brains adjusted so that they would run towards any human with a weapon.
  • The first BGM you hear in the first game is scary enough, considering the dark hallways and flickering lights in the human ship, but the first BGM you hear in the Pfhor ship is much worse.
    • "Landing", "Leela", and "Aliens Again" are also extremely creepy. Makes the dark corridors that much scarier.
  • Did anyone else think the first door you went was unusually loud? It startled this one enough to jump.
  • Some of the message terminals can pretty disturbing. Arther Frain message on You're Wormfood, Dude about how the station has multiple hull breaches is pretty unnerving on its own, but when you consider that the W'rkncacnter is loose and about to munch both the Pfhor and the USEC forces in the area...
    • The Dream levels in general, which, among other things, includes the terminals about the Hangar 96.
    • And this. The fact that these terminals are so nigh-impossible to figure out is about 75% of what makes them so creepy in the first place.
    • The story of Gheritt White. (Third terminal in that level.) It's found at the top of a nigh-inescapable prison cell.
  • Everything about the W'rkncacnter. It spits on the laws of physical reality, seems to revel in destruction and chaos, cannot be killed by any known force, and is never directly seen. But surely, Durandal has at least some idea of how to deal with it, right?.. Nope; for all his power and intelligence, the W'rkncacnter is as obtuse to him as it was to the Jjaro. And Durandal is flat fucking terrified of it.
    • "I'm alri-AAAA!"
    • And their messages are being commed directly into your helmet...
  • A meta-example: One of the scrapped ideas was a weapon that turned killed enemies into zombies. That idea eventually materialized as The Flood in Halo.
  • When a terminal describes the hulks it says that due to their zero body fat they need constant feeding.
  • The "dream" levels in Infinity, especially the messages you find in them, which may or may not be describing the Security Officer's life prior to serving on the Marathon. The first terminal involves him encountering a hulking giant of a man armed with a double-bladed knife; this man proceeds to have a seizure right in front of the Officer, choking out "durability" in-between puking. Watching all of this is a gang of black-suited men. The Officer, as scared out of his mind as anyone else in his situation would be, takes the double-bladed knife and runs for it.
  • The very first message Tycho sends the player in the first game is a desperate warning that the crisis you're attempting to fight your way out of is Durandal's fault—as his code is being picked apart. He eventually gets better... for a given definition of "better"; by the time the second game rolls around, Tycho is a ruthless sadist who can arguably match his "brother" in intellectual fortitude, but has none of Durandal's redeeming qualities. Infinity sees him, in one arc, making his command of the Pfhor absolute by hacking a vital target's teleporter so that it dumps him into space. You had to help Tycho in his treason, by the way. And this isn't the only arc where he strong-arms the Security Officer into working for him; notice how he addresses you as "conditioned unit 7"?...
  • If you're not partial to black humour, then the BoBs' (and Security Officer's) "crushed/immolated" death wails are pretty unpleasant.

Examples of Nightmare Fuel in Game Mods:

  • The mod Marathon: Evil was pretty horror-intensive. In particular, it had the Devlins, a spiny, yellow-eyed, hard-to-see menace that liked dark places and probably gave large numbers of people nightmares. Having one jump out at you for the first time is not calming.
    • And the Mystic Pfhor, their equivalent of the Spht'Kr. Freaky appearance as well as hellish sounds.
  • Marathon RED, from beginning to end. High Octane Cosmic Horror Story. It doesn't help that it's also Nintendo Hard. Intimidating? Maybe just a lee-tle...
    • Paco's back story. In a past life, he braved the anomalous pyramid and unknowingly caught the attention of its 'pilot', Joshua, who was so impressed that he gave Paco his mark. Centuries later, Paco—now among the ten battleroids—tried and failed to defend Tau Ceti IV; when he was rescued from the derelict UESC Marathon, he was so overwhelmed by despair that he attempted suicide...over and over, because he couldn't die. There was nothing Ian's team could do for him except wipe his memory and hope it buried the underlying trauma. Just to twist the knife in further, he learns all this after Joshua has torn Paco's soul from his body to become the Reaver and sent him to tear a bloody swath through his own allies; Ian snaps him out of it, but also makes it abundantly clear to Paco that he's on his own from here on out.
  • Alongside Remixed Levels, this was basically the whole point of Return to Marathon. The original game might be scary, but Return to Marathon overhauls the original game's levels and turns the horror factor Up to Eleven.
    • There are these enemies called "butterflies"—colourful Wasps with pretty wings. One terminal details how they've been breeding aboard the derelict Marathon: by laying their eggs in the dead and not-so-dead. If they don't do this quickly enough, their unborn brood will give their mother a reverse Cesarean.
    • At some point, Egon (one of your two mission controls) notices something amiss in one sector and asks you to contain it by disabling the cores it was detected in. This "something" is a virus that, once freed, immediately infects Egon and overrides his mind in short order. Alaxus, a third AI whose mind is probably buried in limiters, has little to offer except that he also told you to do it.
  • Marathon Phoenix could get pretty creepy at times as well. There's one level in an abandoned mine where if you take a wrong step, you'll fall into a pit of poisonous gas and die instantaneously. It's very dark (being an abandoned mine) and full of mostly silent monsters that can fly, can be released without any apparent warning, and can fire a stream of energy bursts that can drain a bar of your health in less than a second on higher difficulties. The whole level is a veritable fountain of Paranoia Fuel.
  • The swamps on the Pfhor planet in Rubicon have alien noises as random ambient sounds. There are also Lookers in the swamp (whose chatter is among the noises that happen as ambient sounds), and it's next to impossible to see them even if you have liquid transparency enabled due to the thickness of the sludge in the swamp. Needless to say, when wandering through the swamp, you're likely to be constantly afraid that you're going to walk over a Looker and die.
    • To say nothing of the dream levels, the first of which ("We Dream You") occurs near the end of the Chimera plank. You get Haller to safety, Durandal thanks you and warps you out of that submerged hangar, and...huh? What am I doing in the vacuum of space? Okay, I'll just head for this teleporter beam and sort things ou—oh dear. You seemingly end up back on the Chimera, except there's no air (keep in mind that the ship crashed on Pfhor Prime, which has a breathable atmosphere), and no other living beings but these strange balls of light that dash about and get in your way. Quite a few dead bodies and severed heads, though, which you'll pass by as you read through the terminals (which bear Thoth's insignia and continue on from Infinity's dubiously-lucid dream terminals) and try to find your way around. You jump down a hangar, keeping a wary eye on those points of light, and make it outside to a nearly pitch-black box canyon littered with bits of the Chimera and its crew (there's even a set of drawers sitting in the middle of the eastern part of the area). You read the exit terminal and warp out/wake blaring sirens, as Durandal informs you that there's been a catastrophic hull breach. Uh-oh.
    • Over the course of the Salinger Plank, you investigate the Dangi Corporation and the shady goings-on in their half of the eponymous space station. Their big plan is to unleash Achilles, a virus of their own making with a high mortality rate, upon every human-inhabited planet and colony, then offer the cure to the UESC in exchange for total control. Towards the end, their long-suffering research AI, Lysander, reveals to you that his work on Achilles was flawless. As in, there is and never will be a cure, and the Board of Directors has no idea that Lysander is about to jump-start an extinction event.
    Lysander: Once humanity discovers Achilles C15, it'll be too late.
  • An AI Called Wanda starts off by introducing the Security Officer to Freud, the psychiatric AI for the UESC Leviathan; Freud, while a bit on the sarcastic side, is amiable enough (and, of course, makes the requisite "let's talk about your mother" joke). You then take orders from his co-worker Hobbes for a while; when you meet Freud again, he's acting...strange, pontificating about how you're a mindless killer and how such a damaged state of mind must work and feel. His normal dark blue text is stained red. Hobbes soon determines what's wrong: Tycho or one of his clones has spliced his own code into Freud's, resulting in an unstable semi-fusion. The real Freud is still there, begging the Security Officer to kill him, all while Copy!Tycho attempts to beat him down. You can't save Freud; you can only destroy his core.
    • And what of the eponymous Jjaro AI, Wanda? The W'rkncacnter did...something to her that drove her to murder her makers and painfully mutilate the rest. One Jjaro managed to lock her in a maximum-security prison, from which she escaped... ten thousand years later. Having deemed organics to be too error-prone and hubris-ridden for their own safety, she strives to unite them all under her forcibly-modified, brainwashed rule. And while Durandal/Thoth thankfully doesn't answer her call in time, she muses about how once he does, she'll peel back the layers of his mind and see what she can use.
  • Marathon: Fell: Balapoel, who's been aiding the commander of the UESG Tethys for a not-insignificant span of time, coldly turns on them after his rival, Parael, attempts to warn you of his true nature: a formerly-human war criminal.
  • Each of the bad endings in Eternal:
    • Ch. 1: Marcus, under Hathor's guidance, wrecks Durandal's core and takes his primal pattern, so that he can no longer impede the two's progress...or help the crew fend off the Pfhor.
    • Ch. 2: Tycho, who's not quite himself anymore, formulates a plan to trap and eliminate Hathor—that fails miserably. Or did he allow it to fail?
    • Ch. 3: In staying behind to repel the Pfhor from Lh'owon and spare the S'pht from centuries of slavery, Marcus unwittingly provokes the slavers into firing the trih xeem. During the subsequent run through Inti Station, Marcus has his first run-in with the shadowy, deadly Phantasms—without any weapons capable of harming them, mind—and receives a message from none other than the motherfucking W'rkncacnter.
    • Ch. 4: With nothing to prevent Hathor—now horribly unstable from all of her fighting with Marcus—from taking a Jjaro dreadnought, she does so, and heads off to finally make humanity pay for her unwanted state of being.
    • Ch. 5: Marcus shows mercy to Hathor and goes with her to destroy the W'rkncacnter and allow the Jjaro to continue living, instead of completing a mission for the Watcher that would result in trillions of casualties. What he doesn't realize until the end is that the demon that crashed on Earth would never be detected...