[[caption-width:308: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 {{Creator/Bungie}} knew how to scare the shit out of us.

!!Examples of NightmareFuel in [[VideoGame/{{Marathon}} 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 ''VideoGame/{{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 [[FridgeHorror consider that]] the [[spoiler: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.
** "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 ''{{Franchise/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...[[CameBackWrong 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. [[spoiler: 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"?..]]

!!Examples of NightmareFuel in {{Game Mod}}s:
* The mod ''VideoGame/{{Marathon}}'': Evil was pretty horror-intensive. In particular, it had the Devlins, a spiny, [[EyesOfGold 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. [[NightmareFuel High Octane]] CosmicHorrorStory. It doesn't help that it's also NintendoHard. Intimidating? Maybe just a lee-tle...
* Alongside {{Remixed Level}}s, 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 UpToEleven.
* ''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 ParanoiaFuel.
* 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.