This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Warframe
The Infested are made up of Grineer and Corpus alike who were unlucky enough to get infected. The Infested Runners◊ have a new arm growing from their chest and fused to their face and helmets like it was grasping at their skulls, the former Corpus now mutated into eyeless things with no purpose but to get close to an enemy, at which point they spasm and then explode in a spray of gore and nano spores. Others have been reduced to just a legless torso, pulling itself along with its flailing arms. The Infested Chargers◊, once Grineer marines, have been twisted into quadrupeds in the grossest way possible, their chests pointing up in the air with their limbs bent backward and joints inverted, a new "head" forming over their heart while their old skull hangs below their body. As for the Ancients◊, whatever they were before, they have no resemblance to it at all now. Pray that nothing of the original mind remains.
Then there is Lephantis, a bio-engineered Infested monstrosity that was created for the Old War and survived by hiding in a derelict Orokin ship. It consumed and absorbed everything and everyone that discovered it, and so when you reach its chamber, it attacks you with giant, twisted versions of all three enemy factions. The Corpus head is especially horrifying, as the infested Crewman's helmet has fused with his skull and become a mouth that opens to attack you (compare a normal Crewman◊ to the mutated version◊), and the Grineer head has a huge scythe which is basically an instant kill. When you defeat the heads, the floor beneath you breaks, and you fall down to see Lephantis's main body, a building-sized monster with all three of the heads growing out of it. Only then does the boss subtitle appear. Doesn't help that all the while, Lephantis is sending you messages like "We are countless. Consume us. Be reborn." and "We embrace you. Why do you defile us?".
A lot of the horror around the Infestation gets worse when you're familiar with the lore and realize that Warframes are likely based on the same technology as Infested. This is supported by the Excalibur Proto-Armor Skin for Excalibur which has the description, "Discovering the precise nature of this recently uncovered armored prototype has proved elusive. Initial attempts at dating point to ancient origins, perhaps even predating Orokin Empire.". This is a direct reference to DE's previous game darkSector, which had a very familiar threat called the "Technocyte Virus" that would take over hosts and warp their bodies, almost exactly as Infested do. In the darkSector universe it is nicknamed "The Great Plague" and an infested weapon in Warframe called the Mire also references a "Great Plague", hinting that these are one and the same, and Warframe technology was based off of the technocyte virus, and further supported by both dialogue in The War Within calling the Warframe your Operator's "Infested Puppet" and the assertions of Cephalon Jordas in the J-3 Golem mission:
Jordas:"Tenno, it senses you are of like flesh, it is confused. Why do you defile us?" Jordas:"I see it now, you are the same. They fear themselves."
One only has to look at their behavior during an Infested rescue mission to see how smart they are.
At the first day of Warframe, planet Eris was described as the location of a far-flung Corpus outpost. Now, every single mission node there are controlled by Infested. The Cephalon Fragments for the planet describe it as being devoid of all life except the Infestation, which is slowly digesting the mutated ruins of Grineer and Corpus ships.
Occasionally you will find a lone Corpus Crewman stuck in the Infested Meat Moss, struggling feebly to try to get out. You can either kill them yourself or leave them to there to be turned or torn apart by the Infested...
Infested ships are claustrophic, full of deadly hazards that get worse as you kill more hive nodes, near-insta-kill poison enemies can trap you at close range, and there are so many infested in your face that you will start drowning in them if you don't have a decent piercing or crowd-control ability. All that's missing is the M41A Pulse Rifle. To top it all off this music plays throughout. Sometimes an Infested Charger can burst out of a vent next to you, Dead Space style, even if you're on an Exterminate and have killed all the enemies. What's even worse are the Parasitic Eximi that constantly drain your energy and the Mutalist Ospreys which spew toxin fumes that directly damage your health. If you aren't prepared, hive missions can suddenly become Survival Horror.
On some maps, in missions on planets where there is no Infested invasion, you can find in some innocuous corner somewhere a small patch of technocyte Meat Moss hidden and growing. No planet is truly clean; the Infested have footholds everywhere and will always, always return. The Origin System will never be free of them.
There's more to the Infested room on the Orbiter than just the chair and cysts. Something named Helminth resides down there, and it apparently serves Nidus. It wants Nidus to consume with it (fitting, given his territorial abilities.) As disturbing as it is, there's no getting rid of it since Ordis says it controls the biological functions of the Orbiter, but even he's disgusted by its existence.
As of the Pacifism Defect event, we get to see in real time what it looks like when someone succumbs to the Infestation, courtesy of the Grineer defectors in the Infested ship. First, they collapse like anyone does when they're out of health. Then, if no Tenno revives them in time, they're consumed by an Infested spawn pod, which creates a Charger. This is treated unambiguously as being a form of death for the Grineer.
The Grineer are a near-limitless horde of decaying clones that - aside from a few individuals mentioned below - possess two major psychological traits: Fanatical loyalty and love for their evil queens, and an equally intense hatred of everything that isn't them. And these traits are inherent to the Grineer as a whole. Sympathetic Grineer such as Clem, the Steel Meridian members and the Kavor defectors are all considered by their empire to be mistakes that need purging — their functional consciences are explicitly described as a defect in the cloning process. Grineer are designed at the genetic level to be psychopathic killing machines. They have no rules and restrictions on warfare, targeting civilians just as often as they do combatants. Anyone they don't murder is rounded up and forced into slavery.
Unlike the vast majority of mass-produced armies in fiction, they aren't weak, fragile incompetents that can't hit the broad side of a barn. Each soldier is a extensively-trained professional killer, the vast majority of which are heavily armored with strong alloy plating. Their bodies are heavily augmented at the genetic level for stength, endurance, and durability. They use extensive cybernetic implants to increase their already formidable capabilities and shore up any physical flaws, while also linking them to their - very effective - weapons in a way that lets them wield those weapons like an extension of their own body. Lore-wise this makes every Grineer soldier a crack shot with their guns, as well as highly adept at hitting you with melee weapons in ways that will do the most damage. And they have more reservesthan you would ever believe possible.
Vor's ominous threats and taunts as his Ascaris slowly takes more and more of you over in Update 14, culminating in this line that replaces the Lotus' standard "Mission complete" dialogue at the end of the Corpus Hijack mission.
Vor: You are MINE.
Running high-level Void missions reveals that after you killed him, Captain Vor didn't stay dead and is now loyal to something that dwells within the void
Grineer Manics. You're just going about a mission, happily killing things until you start hearing this insane laughter and then these things show up. They can be seen as mini-Stalkers, seeing as they can turn invisible and invulnerable, can regenerate their health, can Dispel your abilities, and can easily kill you if you're not cautious.
The Grineer Blackout tactical alert. There are no enemies until you reprogram the navigation computer. Then 75 Grineer Manics appear out of nowhere...
Before the Grustrag Three show up, the Lotus contacts you about 'impossible readings' she is picking up nearby, and her transmission is abruptly cut off. She gets out a few more garbled transmissions about how you are 'being hunted' and then suddenly begs the Tenno to abort the mission now before she is cut off totally. There's a few seconds of nothing, then the Three spawn into the level. If you aren't equipped properly, they will beat the snot out of you, trap your Warframe under an orange cage-thing, and fix it with a Restraining Bolt that makes it almost impossible for you to effectively fight other Grineer. The genuine concern you can hear in her voice is what really hammers home what a threat the Grustrag Three present to the Tenno. While the Lotus obviously cares about you, she is nothing but utterly calm or mildly surprised at most, even on the most dangerous missions. Suddenly, she is clearly frightened and confused, frantically warning at the player to abort and run before getting cut off.
The War Within, while grim, hides it's horror more subtly. For example, the Queens know exactly who you are, and about Ordis. The Elder Queen also states the reason why her possession didn't work. She spends the quest slowly but surely corrupting you, culminating in you moving as she does.
Elder Queen: I see how you cast me out. Simple really. There's no room for me in that head of yours.
In the Pacifism Defect event, the Tenno are tasked with escorting Grineer defectors from Sargas Ruk's army (who aren't trying to join the Steel Meridian, but just want to live peacefully) out of ships that have been hit with the Infestation. When most Grineer speak, their voices are rough and almost monstrous, distorted by their cybernetics. Not so for these defectors. Despite not having their dialogue appear as text on the screen (which makes them feel more like regular Grineer NPCs), they sound like completely normal human beings. That makes it all the more nightmarish when they start dying to the toxic air in the ship. Their desperate screams are haunting.
Kavor Defector:TENNO, THIS IS KILLING US!
In the higher-level missions of the Pacifism Defect event, Sargas Ruk begins sending in Grineer Manics just in case the Infested don't kill enough of the defectors. It doesn't matter that the escaping Grineer simply want to live peacefully and will pose no threat to anyone once they're out: their superiors still want them dead. Life as a Grineer subject seems bleaker by the minute.
The Zanuka Project: various sliced-up Tenno and their warframes formed into a mechanical quadruped controlled by Corpus technology. What the hell this does to the unfortunate Operators on the other side of the Warframes in question is unclear.
Valkyr, the Warframe produced as a byproduct of the Zanuka Project. All subsequent Valkyr frames are based off of the template of the original. It's impossible to tell what frame Valkyr used to be, or if we've even seen it yet, but that's not even the scariest part. Given that Warframes draw their power from the Void in a manner that is suggested to be intrinsic to their psychology and outlook on life, it suggests that whatever Alad V did to Valkyr caused her psychology to change so drastically it altered her Void imprint; and knowing what we do now, the implication seems to be that Valkyr's original Operator was linked to it during the entire procedure, which seriously starts to approach Asuka Soryu levels of characterhorror.
Valkyr's original (non-Prime) form is demonstrated by her Gersemi skin. With it on, she looks like a fairly normal Warframe. There's just one little detail: The Gersemi skin bears an uncanny resemblance to that of Zanuka. Now we know where Zanuka's outer layer came from.
"The Profit" Trailer begins with Alad V cutting up an Excalibur, then preparing to do the same to a Mag. It's unsettling enough that Alad maintains a friendly, affable veneer throughout the whole thing and it's unsettling enough that he treats the whole thing like an auction (with prospective buyers!), but knowing that the Excalibur he cut up is still connected to his Operator - and knowing that that is true for every Warframe he cuts up for the Zanuka Project - adds a horror to the whole thing that puts it up there with what happened to Valkyr.
Ever wonder why there are so few female Corpus mooks? Corpus crewmen are described as 'purpose-bred'.
Story-wise, what little information we have of the Sentients leans them towards this. These are something so threatening that the Orokin unleashed the Technocyte Plague and the Tenno in order to try and stop it. They pilot "worm-ships" which use heat-bursts to boil their enemies from the inside, are described as having "multi-faceted eyes", and are nearly endless in number.
The Tenno were chosen to be used in the war because their powers were spawned from the Void— a "hellspace where science and reason failed"— and thus couldn't be subverted by the Sentients, who were getting stronger with every advance the Orokin fielded. Lotus reinforces this in Vor's Prize, pointing out that the presence of the melee weapon was an adaptation to the Sentients' ability to subvert technology.
Ember Prime's codex entry implies that the Orokin were so desperate that they even threw kids into the Void. It was more an accident, really, but the effect was the same.
You have read all of the entries here, yes? The Sentients are returning. Unless you've seen the trailer for Tombs of the Sentient, you won't know when Teshin, the Conclave master, references them. All you'll know is he's trying to prepare you for the evils returning from beyond the outer terminus. Need we remind you again that the relic you interact with in The Second Dream quest is the corpse of Lotus' father, Hunhow? Yes, that one gigantic pillar stuck underwater allllll the way up to the surface of Uranus is only a part of it!
After competing The Second Dream and going back to Lua, you can run into Sentient Oculysts that can then summon either Sentient Battalysts or Conculysts that can adapt to your attacks just like they did in the Old War. The screen flickers blue like as if an Assassin is spawned and a loud metallic screech sounds that signals the appearance of both the Oculysts and their Sentient fighter backup, and while you fight them the Background Music is replaced with a hollow-sounding wind.
The Shadow Stalker first appears as The Second Dream's main antagonist and takes the Stalker and turns him into a pawn of the Sentient Hunhow, replete with frightening armor attachments and Hunhow's sword, War. His appearance at the Climax of The Second Dream when he confronts the Operator within their Orbiter's Somatic Link, stabs their Warframe with War, and attempts to strangle the Operator is one of the most frightening sequences in the entire quest, and Hunhow's dialogue only ups the tension. After completing The Second Dream he will also replace the regular Stalker and while he's not as much of a threat since he doesn't have his bow, throwing knives, or Sinister Scythe, he still has War and he also has the Sentients' ability to adapt to your attacks.
The Sentients are capable of subverting all forms of Orokin technology, at least if it's not touched by the Void. This even includes Cephalons from the Orokin Era, namely Cephalon Suda. And then, because these Cephalons are interconnected, they can spread their mind control to other Cephalons. And even better, by controlling only one Cephalon, they could have gained the power and resources of a enitre Syndicate, with only you, Ordis, and Simaris the wiser. And they nearly kill Ordis and the Operator as well.
In Plains of Eidolon trailer: a gigantic structure of Sentient origin lies dormant near a civilian colony. Then it moved.
The Tenno themselves. Almost completely silent killers, who can strike almost anywhere and at any time. Their nature is largely unrevealed as well. All we know is that those who made it back from the void came back wrong. High ranking Tenno can and will single-handedly wipe out entire armies, from lowly grunts to the elite of the elite. The only enemy who regularly gives a well equipped Tenno pause is the Stalker.
What happened to the original Limbo. Specifically, the way it's described to us.
The Tenno are Mook Horror Show incarnate if you think about it. They're terrifying and unstoppable One Man Armies, each with their own personal style of slaughter. From Volt's Shock and Awe, Ember Playing with Fire, and Saryn being a Poisonous Person they're all uniquely terrifying. They fly around in stealth ships and can show up anywhere. From the most isolated outposts to secure fortresses, at any time. And they are nigh-impossible to stop.
With just a competent 4 man team, each Tenno can rack up 300 kills each, in less than 20 minutes against an army of trained killers. In half an hour, a Tenno can kill close to a thousand Grineer during a survival mission.
The War Within gives us details that definitely fall into this category:
The Zariman Ten-Zero incident.
The Elder Queen: Paranoia gripped your father's mind. What was it he said - as he stared out into the starless black? The Operator: Something's out there kiddo... watching us. (Later in the quest) The Elder Queen: You remember then, how the howling stopped - they had broken through. The Operator: (Moon) They were nothing but animals... so I hunted them.
After defeating the Grineer Queens and unlocking all of your Operator's Void powers, you meet Master Teshin on the same snowy peak where you fought your Battle in the Center of the Mind against the Elder Queen, still in possession of the Kuva Scepter you stole from her. After conversing with Teshin about what to do with the Kuva within the scepter, you get a choice: Destroy the Kuva (Sun Alignment), let Teshin Control it (Neutral Alignment), or Consume it (Moon Alignment). Whichever choice you go with, your Operator is suddenly controlled by an entity called "The Man in the Wall", who is implied to be connected to the Void and the Tenno's powers. Their eyes turn deathly black and the background darkens while they deliver an utterly chilling line:
Rap. Tap. Tap. Chains of Harrow is, to put it scientifically, creepy as fuck.
It starts with a playback of a Steel Meridian transmission, during which the screen subtly turns blood red, then fades to black, and an electronic groan is heard before the screen returns to normal.
After the end of the quest your Operator will be happily sitting in the cockpit to greet you after freeing Rell. But then you realize that you can still control your Warframe...meaning that isn't you. If you walk up to it, you hear something you never would have wanted to hear a second time:
After that happens, the being disappears as soon as you turn away from it. And after the first encounter, there's a chance that the Man in the Wall will randomly appear in your Orbiter after completing a mission, delivering its line and summarily vanishing. There's no way to tell where or when it's going to appear.
Everything you've been doing, gaining your strength to fight the Queens, releasing Rell... It's only making The Man in the Wall stronger. As the Tenno grow stronger, so does he.
Despite the Lotus's insistence that it's just a delusion, The Man in the Wall seems to be becoming more active.
The Orokin. Just the Orokin. Each new bit of lore focusing on them reveals a new, incredibly disturbing level of dehumanization, scientists with Morally Ambiguous Doctorates, and incredibly unethical experimentation; which gets even worse when it's revealed that the Tenno are Child Soldiers who haven't aged a day since they entered cryostasis centuries ago. The Orokin considered it acceptable to take casualties from a horrific Teleporter Accident and turn them into soldiers just to make use of the destructive powers the Void gave them; to the point where they killed their Parental Substitute to make them even more efficient. These silent masters of warfare, capable of slaughtering absolutely anything in their path, are less than seventeen years old when they finally wake from stasis. And they've experienced all those battles as if they were there in person, thanks to their Warframe proxy units.
The Void. Pristine. Beautiful. Transcendent. Utterly empty except for "corrupted," versions of enemies you face in normal levels turned into mindless drones by dying in the Orokin Towers they invaded and being turned by these Towers' Neural Sentries. The music that plays here is simultaneously serene and unsettling. Look out a window and all you'll see is black dots in an endless white expanse. It begs two questions when you first set foot in them - who were the Orokin, and where did they go?
In Specters of the Rail, we finally learn of Ordis' origins. He was originally a soldier/mercenary named Ordan Karris, who fought for the Orokin for so long and so viciously that he became known as the Beast of Bones. When he was brought before them to honour his service, he used the opportunity to kill as many of them as he could. Unfortunately, they had planned for this, and his assassination attempt failed. As punishment for his treason, they subjected him to a process that converted his mind into a Cephalon. All parts of his mind that weren't needed were locked away, and any thought of rebellion was suppressed by what is implied to be a neural sentry. After the fall of the Orokin, the sentry broke down, leaving Ordis free to remember. He ended up locked in a loop of trying to recall his past, then wiping his memory in horror when he did. Of note is the weapon he uses to carry out his plan. Known as "Bone plugs", they are hooked fragments of bone that are embedded in the wielder's neck, wrapped around the superior vein. When used, the wielder pulls them out to use as daggers, so they only have a few seconds before they succumb to blood loss.
The nature of Orokin immortality. Orokin reaching a point where their bodies are on the verge of death go to Yuvan theaters where the young and exotic - often no more than children - are paraded and sold like slaves, to be used as those Orokin's new bodies in the "Continuity" ritual. How do they do this? By Mind Raping these kids until they're little more than an Empty Shell that the Orokin can freely insert their minds and souls into. And you learn this during The War Within, while you're fighting a Battle in the Center of the Mind to stop one of the Grineer Queens from doing exactly that!
Mild one in Plains of Eidolon trailer, where the villagers going about their daily lives, dismantling what seems to be... flesh... from an abandoned Orokin structure... Just what did they use to build it is anybody's guess. Which also raises the question: Is it possible that other Orokin structures and machines are like this? Are the Orokin Towers made of this? Your sculptures? Your ships? Your weaponry?
The Stalker, if you aren't expecting him. You're out, minding your own business, celebrating a boss' defeat, when suddenly, the lights flicker. Then:
Stalker: [Player], you cannot run from your past.
During the Second Dream quest when the Stalker put his War greatsword through your Warframe, he was looking at the Operator the entire time. Then there's the fact that the Warframe that was just described as a mindless machine, takes War, the current source of the Stalker's power, and breaks it.
The premise of The Second Dream. You spend quality time with your nice and cozy Warframe then you get decked back out into your unfamiliar body, dazed and confused, while a dozen Sentients are out for your blood...
Reaching the lowest reputation possible with New Loka makes them suddenly dial the creepy-factor up to eleven if you then visit their syndicate menu. Given their flavor text and some of the rather unnerving statements New Loka's leader says even when she likes you might point to a much darker intention. Some players even wonder if New Loka might target the 'impure' Zariman children once the war is over if they ever found out what the Tenno really are.
Amaryn: You're just like the rest. Tainted and ruined beyond salvation.
On the relays, some of the Red Veil are preparing to vivisect a captured Grineer. Also, they have several infested in a cage, for unknown purpose.