The much greater detail in which we see blood-stained walls, and eviscerated corpses littered everywhere. While these were present in classic Doom, the low detail gave them a surprisingly cartoonish Black Comedy tune; very much not so in the new Doom.
The Revenant's shrieks, period. It sounds positively tortured.
Hell, the Revenant itself, especially if you read the UAC Handbook. The Revenant just looks scary to begin with, being a gore-smeared, enormously tall skeleton with Multiple Launch Rocket Batteries on its shoulders, exposed entrails danging from its torso, and staring, blank white eyes in their sockets. But the thing is, Revenants used to be human: the UAC "upgraded" them as part of a Super Soldier project that involved mutilating them into their present state, whilst they were still alive, and then exposing them to a weeks-long barrage of Lazarus Wave blasts until they finally expired from the excruciating pain. Then the energy they were exposed to brought them back. No wonder they scream...
They used to be human, but now everything considers them as much of a demon as pure-bred hellspawn like Barons.
A security hologram of a Hell Knight ripping an unlucky Red Shirt in half before smashing a computer technician into the keyboard, crushing him, before turning around and snarling. Pretty good way of setting up the creepy atmosphere.
Adding to the creepiness is the way that it stares in the direction that you will be standing in order to activate the hologram playback, as if it knew that you would be there in the future.
Some of the weapons are taken off various corpses with a small but noticeable amount of effort. How terrified were the former owners to have gripped their weapons so hard?
The Doom Slayer's ability to dish out Finishing Moves is pure Awesome... but the demons can do it, too.
The humble Imp has a surprisingly nasty one; after knocking you over, it then rolls you over and tears your guts out with one hand. Then it grabs your left arm, rips it off and proceeds to beat you to death with it.
The lowly Possessed/Unwilling have a shockingly visceral one: Ripping out a massive chunk of your throat with their teeth.
The Cyberdemon can launch a cutting attack that, if it kills you, will result in the Doomguy falling to the ground, after which you see his still standing body falling down to join him.
The Cacodemon's melee attack is a bite so wide and fast that it instantly tears open your ribcage and abdomen, leaving Doomguy able to stare down into his own intestines before falling over.
The Doomguy himself became this! He just regained consciousness and the first thing he does is smash a possessed scientist's head in and shoot a few more of them before he even puts on his armor! In-universe, he legitimately terrifies the legions of Hell, as he has rampaged through their ranks for a long period of time and even slain one of their largest and mightiest champions, whose bones still litter their lands. The opening perfectly sums it up.
They are rage, brutal, without mercy. But you. You will be worse. Rip and tear, until it is done.
To paraphrase 4chan: Why doesn't The Doomslayer have PTSD? Because he is the traumatic event.
You think the Pinky was fairly easy to handle in the original game? Now he comes with armor-plating that deflects your frontal attacks - and they're too big to simply double-jump over when they charge to devour you. Enjoy...
The Zombie Marines of the previous games were bad enough, the idea that Hell can commandeer your corpse and turn you into a weapon to hunt down your living friends, family and allies is pretty scary. But Revenants are worse: they volunteered to be killed, mutated and reanimated with the Lazarus Wave procedure, all for the glory of the UAC...
The Possessed in general. As the page image shows, the mutations caused by Lazarus Wave exposure are not pretty. The Lazarus Wave seems to melt and burn items into your flesh- Possessed Soldiers, for one, have melted into their armor, and their Plasma Rifles have become meshed Arm Cannon style weapons totally replacing their lower right arm.
The UAC is pretty damn scary when you think about it. This is a megacorp that, upon finding out that Hell actually exists, decides that the best course of action is to exploit it in classical 20th-century style. They start by harvesting the seemingly magical "Argent Energy" from Hell, using it to pioneer advances in cybernetics and artificial intelligence. Then they turn to mutating their own military forces, creating the Possessed and the Revenants. Then they start capturing demons and trying to turn them into warrior-slaves — the Spectre's invisibility? The Cyberdemon's implants? The Cyber-Mancubus's extensive upgrades? All designed by the UAC, to make them better killing machines. Did we mention that nearly all of the higher tier members of the UAC are part of a demon cult run by the fanatical Olivia Pierce, and that the higher you rise in the ranks, the more indoctrinated you become?
Making things worse is that, the more you think about it, the more you wonder why the UAC started trying to make soldiers out of Hell's creatures. What could possibly be out there (besides, well, you) that makes weaponizing demons to fight it seem like a good idea?
And it gets better. The UAC's advice for dealing with a demonic encounter, if there's no escape and your equipment has been powered down? Give yourself over willingly to the demon so that when it kills you, it'll leave enough of you behind for post-mortem study. Notice the specific "powered down" bit- plenty of Fridge Horror when you realize the UAC would have no issue with doing just that to an unlucky soldier's armor if they thought they could get science out of the result...
It gets even better than that. The primary purpose of the UAC's Mars facility? Argent Energy, siphoned directly from Hell. It surpasses everything humanity has utilized prior - and after exhaustion of Earth's fossilized and fissionable fuel reserves it is the only thing keeping Earth and her colonies from collapsing right into pre-Industrial Revolution levels of society or worse. You destroyed the Argent Filters, because as the Sole Survivor of your homeland, you know exactly what comes of messing with anything involving Hell.
The horrendous shrieking sound that Gore Nests make when you destroy their heart, kind of blurring the line on whether or not these things are alive.
It's apparently countless demons screaming in pain and/or rage. Imagine how many demons are linked to these things.
If you're wearing headphones while in Hell, you'll often hear unseen creatures scurrying around you and disembodied voices speaking in an unknown language.
It's implied that those who live in the Argent Facility eventually develop serious diseases, requiring them to resort to Argent Energy-powered cybernetic implants to survive. Olivia Pierce came down with acute idiopathic scoliosis, while Samuel Hayden came down with brain cancer.
As the game nears its end, the scenery and atmosphere changes dramatically. The mundane labs and foundries are now covered in glowing pentagrams, ominous candles, and "trees" of flesh. The UAC spokesperson continues his cheery speeches... while speaking in tongues! Even the loading screens turn from relatively generic computer-generated overviews of the next area to images of your destination accompanied by unholy symbols and demonic letters.
The improved graphics engine really helps Hell come to "life". Now the pools of blood, grotesque architecture, walls lined with skulls, and horrific torture devices are rendered more realistically than ever!
To some, it's cheesy and pure Nightmare Retardant due to sounding like something out of a metal album. To others, the mysterious Demon Voice that narrates the Slayer Testaments as he details the Doom Marine's one-man slaughter across Hell and other matters related to him. But he's also the first to comment on your arrival in Hell, and remotely commands the demons to stop you in the endgame, implying there's an almost omniscient demon god out there watching everything in Hell.
That same voice is threatening Olivia right in her office in the Lazarus Labs about how the Doom Slayer was found and released. In a situation where you only see her standing there in an Echo flashback, you get to see her eyes bulged wide in horror at the mere thought of having displeased this mysterious demon lord as she pleads with the voice to give her another chance. You have to wonder what kind of horrible fates this demon lord has in store for those who fail him, which would give even a fanatic like Olivia reason to quake in her boots.
Lost Souls initially float around relatively docilely, bobbing up and down while being covered in a blue flame. However, getting too close to them will result in the Lost Soul CAREENING towards you accompanied by a horrifying shriek. In some places of Hell, you may not notice them until the screams are essentially in your ear.
The Foundry level is a mix of platforming above pits of damaging molten steel, and being chased by the extremely fast Hell Knights, which at this point you are woefully under-equipped to deal with, through tight and poorly lit corridors.
The Spectre's codex entry picture looks like something you'd see during a sleep paralysis episode.
Even the power-ups are more than meets the eye, as the Codex explains.
Berserk, which allows you to One-Hit Kill all enemies in the game save for the Hell Guards, The Cyberdemon and Olivia Pierce, causes the subjects to rip and tear themselves if they cannot find anyone else to kill.
Haste gives the Doomslayer Super Speed, but causes the subject's heart to undergo a "myocardial rupture", i.e. it fucking explodes.
Invulnerability makes you invulnerable by applying a layer of plasma energy to the Doomslayer's skin cells, which is an incredibly painful process and causes the subject to go mad from the pain and in turn fight enemies that are far beyond their abilities.
Quad Damage, one of Id's most iconic powerups, was revealed in Doom 4 to make normal subjects go Ax-Crazy.
Granted, the fact that none of these side effects happen to the Doomslayer make this pretty awesome instead.
The secondary Codex entries for each environment are essentially an employee handbook for Pierce's followers that start rather darkly funny, but gradually become more sinister, ominous and chilling until you finally read the last one, titled "A Farewell":
Some of you in a previous life may have held childish notions of God and the kingdom of heaven. This ugly representation of a spiritual nirvana was designed to control you, to placate your natural feelings of hate and rage, to subdue your realization of your place in this universe. By embracing science, you took the first steps towards freedom from these intellectual chains. By dedicating yourself to the development of Argent Energy, you cemented your place in the next age.
Now let us tell you about the real heaven.
The real kingdom of the gods is a place that you will never reach, and you should never wish to. No human will ever visit the sacred ground of Argent D'Nur, unless they are made a god by the dark lords. Humanity's only purpose, and reward, is to serve the ascension of the Imperatrix and protect the Crucible. You will be destroyed by the demons, while I will be made a god. My immortality is assured, while you will writhe in perpetual agony at the bottom of the darkest hole in Hell. You will be forgotten. Your life, your loved ones, your achievements and failures are nothing. A blank space on the canvas of time.
The VR spinoff, Doom VFR, is generally pretty terrifying as you're a more rank-and-file human as opposed to the goddamn Doomslayer. By far the worst part however is the opening as you get to experience the Hell invasion first hand as a scientist in an elevator when the surge hits. What's waiting for him when the door opens? A Pinky which proceeds to rush and kill him. The only reason the player survives at all is someone uploaded them into a robotic shell. The first thing you see when you wake up? Your own gnawed on corpse.