The face in the HUD also keeps getting bloodier as you take more damage. In some versions, a big hit when low on health makes the head explode.
Daisy! The Doomguy's pet rabbit, with its head severed and impaled bloodily on a stake, as shown at the end of Episode 3 — and for those who played the original version, this was the end of the game!
The Spider Mastermind. A creepy-ass demon with a brain that's far too big, on a mecha platform with four spiderlike legs and a powerful chaingun that will tear you apart if you're too close.
And in Doom 2, you find out that Spidey is a mommy — and her little Arachnotron babies are every bit as bad as her with their plasma guns.
In Doom 2's map "Dead Simple", once you killed all the Mancubi, the walls would come down, surrounding you with hordes of Arachnotrons.
Doom 2: at the end of the first major episode (MAP11), the room containing the master control switch has walls entirely comprised of stitched together corpses.
The "pantry" you encounter near the end of Doom 2's MAP27 (Monster Condo), which is filled wall to wall with hanging and skewered corpses. The level's eerily somber music makes it that much worse.
Despite their relative weakness compared to other monsters, the noise made by former humans as they wander around is quite disconcerting.
The scream the Big Bad of Doom 2 makes when you fire a rocket into the hole in his head that the monsters come from.
And that's only the half of it: as soon as the noclip cheat reveals the TrueBig Bad, whose head is actually residing in a small room, while its' hanging on a stick... Hey, the hint to the beast's real identity is even here, in his only spoken quote (consider reversing that):
Oremor nhoj, em llik tsum uoy, emag eht niw ot...
There's an example from one of the Final Doom level sets that both qualifies as Nightmare Fuel and makes the developers into Magnificent Bastards. The level starts in a thin hallway with other, shorter ones protruding out from the sides. Walk to the end of one hallway, the wall behind you opens to reveal another hallway, this one filled with monsters. Clear them out, go down the next hallway, sequence repeats. When you go down the third hallway, you've probably caught on by now, so you go down that one backwards... and then the hallway extends and you still get ambushed from behind.
MAP11 from The Plutonia Experiment. Start off with the music from the ending of the original game, see a backpack and a switch in the room ahead of you. And then you look to the left and see a ton of Archviles frozen in place behind a fence - and when you hit the switch, they un-freeze and teleport out into the maze you're just about to enter. The level is quite fittingly named "Hunted".
It's bad enough that the red key maze in "Halls of the Damned" (E2M6) is nearly pitch dark, but it gets worse when you realize that it's dotted with Monster Closets, which open with no warning. (Of course one can always grab a nearby pair of lightamp goggles, but that would be no fun.)
The side maze that eventually leads to the chainsaw in "Nuclear Plant" (E1M2) is very dark with fairly erratic lighting, not to mention the unsettling moaning of former humans as they search for you.
The final level of the first episode of Doom, "Phobos Anomaly", ends with your marine stepping on a teleporter that plunges him into a black abyss where you are constantly losing health either from simply touching the floor or being assaulted by a barrage of barely-visible monsters. You stay there being ravaged by these things until you run out of health, at which point the next episode begins. Truly becomes Nightmare Fuel if you only have the Shareware version as this is where the game would end, leaving you to wonder what exactly happened to your hero when he stepped on that teleporter.
DOOM 64 remains less well known than DOOM 1 and 2, but it manages to take the classic DOOM gameplay and make it much scarier, with darker graphics, more muted and grim colors, and one absolutely creepy soundtrack. Gone are the heavy metal inspired tracks, replaced by incrediblyunsettlingambientmusic
And those aren't even the worst of it. You want the definition of unsettling ambiance? Here's the theme for Breakdown. Have fun sleeping tonight!
Doom 3 and expansions
After Hell invades, go to the bathroom in first part of the game and look in the mirror. A Jump Scare follows as your face momentarily turns demonic and the camera zooms in on it.
Cherubs: Babies with wings and lower bodies of insects, often traveling in swarms and making creepy baby sounds.
The Trites are flesh-coloured spiders the size of sofa cushions with an upside-down human head instead of a body, with eight black eyes to make extra sure you don't get any sleep. They weren't too bad for most of the game but there was a Jump Scare late in the game involving a touch-screen computer and a nice dark compartment behind it for a Trite to hide in.
At one point in the game there was a flight of stairs with a landing covered in blood (the walls, the floor, just drenched with it completely). By this point in the game that seemed like absolutely nothing. But when you walked out onto the landing what appeared to be a face suddenly appeared on that blood covered wall.
The introduction of the Lost Souls. Before you fight them, you hear a woman crying. When you see her, she weakly tells you to help her and she turns around. You get to see her skin greying and her skull forcibly ripped from her head along with her spine.
The scene where bloody baby footsteps appear on the floor, and a ghostly voice implores to "follow me" and "hurry". Eventually, it says "they took my baby", and the screen turns red for a moment as creepy baby cries are heard.
Near the start of the game there's a segment where you're crawling through an air duct, when all of the sudden a ghostly voice whispers "Help me!" and it sounds like it's right next to you. If you happened to be wearing headphones the first time you played that part you probably didn't sleep with the lights off for a while.
Upon traveling through the first part of the Communications Facility, you come across a door drenched in blood, with dark red streaks across it. When you first open the door, a demonic face bursts through the fleshy wall behind the door and growls hideously, before receding back into the folds of the flesh, as the door closes permanently.
This troper found out the hard way that using your PDA doesn't pause the game, it pauses you. Remember that when you use it and suddenly hearing rapidly advancing footsteps...
Less so with the original Doom, but the Aliens custom mod is certainly scary. When you're holding a Pulse Rifle, and there's a dark corridor lit only by a sporadically blinking light, and you hear strange sounds coming from potential corridors in that darkness, willing yourself to press the "forward" key is an exercise in futility.
This is a video of a grown man screaming like a little girl while playing the Doom 2 mod Ghoul's Forest 3. Ghoul's Forest 2 and 3 are quite possibly the most terrifying mods for Doom 2, for that matter. The entire concept is that you're hunted through a dark forest by giant floating super-fast monstrous heads who pop up unexpectedly and kill you in seconds.
The backstories of the ghouls are creepy: the Big Bad Yurei is the ghost of a pissed-off girl who was abandoned in the Forbidden Forest. The Creeper is the spirit of a baby, the Sjas was a psychotic jester who was executed by the king for a certain joke, the Jitterskull was a Giant warrior, Choke was his demented brother who killed himself. Frostbite (who only appears in the multiplayer mod Ghouls Vs Humans) died of frostbite, so now he can breathe ice and swallow people whole like the Jitterskull.
The mod is made worse with Icy's addon, which adds a clown; a skull that makes the Jitterskull look like a joke; and a Sjas-like mist cloud that is absolutely relentless. These new ghouls use the terrifying creeper scream. There's also a chance that, instead of the Yurei, the final boss will be the entire cadre of ghouls at once.
The Skulltag Armageddon 2 multiplayer map pack is infamous for the "By Day, By Night" level. It's a map that alternates between day and night. By day? Cheerful demon massacring, upbeat music. By night? The entire map goes dark, the music turns into horrifying loomy ambient, and the monster are... different. And the Eyesores appear. Eyesores are ghouls, just like the ones in The Ghoul's Forest. Imagine a demented, deformed face on spidery legs which runs around like it's on crack, makes weird "kekeke" sounds and can kill you in seconds. And there's an entire crowd of them attacking you all at once.
The final boss of the map: Clowny. An invisible face which flies around the entire (big and open) final area and pops up suddenly in front of a player for an instant-kill and a Jump Scare. And sometimes it somehow ends up inside the mazelike building you've been in before, forcing the players to brave its silent corridors, expecting the bastard to jump out from behind every corner...
The Happy Time Circus series is the very definition of horror, especially the second one. Wandering through an abandoned ghost town with lots of silence and empty moments, only to have an area completely enveloped by Monster Clowns that you may or may not be able to handle? Oh Crap...
The areas that take you through Clown Hell (which is also slowly encroaching on the town itself) are no less horrific. Bright, cheerful, flashing colors in extremely low light conditions produces a highly unsettling effect, and the heavily distorted circus music sometimes heard in these places doesn't help one little bit. And as you might have feared, Pennywise Itself makes several appearances throughout both mods, and he's a boss-level monster in the first one.
In the midst of all this, you'll also need to escort "Fluffy" back to Clown Hell. Fluffy is a fast-moving, unkillable severed rabbit head that can pass through walls to chew off your face, and you have to outrun him through a narrow passageway in a dense thicket where the tiniest misstep will allow Fluffy to gain on you. Good luck, and keep some extra underpants; you will most certainly need both.
The Equinox mod, at first, might make you do a double-take when you realize such a well-made mod is from the same guy that made the infamous "nuts.wad". Then, about map 13 or so, you infiltrate an alien space ship. It is incredibly dark, very cramped, the music is creepy, and it is filled to the brim with Arch-Viles.
Unloved is the mod that you always wanted to play but hoped no one would ever make: it's a high-res megawad that borrows much of its atmosphere from the Silent Hill series and it's exactly as terrifying as that description would suggest - while keeping the gameplay of Doom alive. If you ever wanted to prove people who insist horror games should be weaponless wrong, this .wad is it. note Use of Unloved with the source port GZDOOM only is highly recommended, as other source ports do not offer the lighting effects that set the perfect mood and can lead to severe Nightmare Retardant from the hundreds of brightly-colored "!" sector flags they leave visible in the game.
While the first The City of the Damned map is plenty spooky and atmospheric enough, the second installment "Apocalypse" is more horrifying in every possible direction. Imagine the game Blood with all elements of Black Comedy stripped out, now add in chilling Apocalyptic Logs scattered about that tell the story of what happened to the town and several frightening scripted events like the ominous sirens that sound off now and then. The creepy soundtrack to Resident Evil 3: Nemesis is also put to very good use throughout the mod. Oh, and those sirens? When you hear them, stop what you were doing and get to a shelter. FAST.
The (sadly unfinished) Ghostbusters DoomGame Mod for Doom, which recreates scenes from the movie, fulfills this trope with its first level - the haunted library. While the original film scene was quite amusing, its recreation in the mod is much creepier.
And then three levels later, you have to investigate a haunted mansion, in the rain, with creepy music, and corpses everywhere, and the corpses start standing back up. They take way too much punishment just to shoot off one of their arms - you have to deal just as much damage a second time to get one to stay dead.
Eternal's Epic2 wad is a collection of egyptian themed levels, city levels and spaceship levels. One of the spaceship levels, level 13, takes place in this very eerie, dark, grey and white spaceship made of what seems to be rotting tissue. The soundtrack for this level is definitely creepy, very different from the others. The battles are exclusively with arch-viles (arguably the creepiest enemies in vanilla Doom), most of them used through teleport-traps. Health pickups? Aside from a secret soulsphere, the only health pickups are accessed through computer panels marked with a scarab, restoring 20 health each.