This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Resident Evil 5
The fifth game gives us Uroboros. Imagine a writhing mass of worms tearing off someone's skin from the inside out, leaving only the head and limb muscles and bones intact. Now imagine this mass of worms chasing you with the intent of absorbing you, dead or alive, from the outside in....
Fun fact: the name is from the Greek "Ourobouros", which normally refers to the symbol for infinity of a snake swallowing its own tail. It literally means, however..."self-devourer". Yeah. Accurate, huh?
The Adjule, Plagas-infected dogs. Not only do they look like some kind of mutant greyhounds, but then when one takes enough damage their FRIGGIN' HEADS SPLIT APART! Their entire body from the shoulders up has now become a gaping maw, complete with teeth and tentacles coming out.
Also, when they kill you, you get bitten in half. Messily.
Later in RE5 you get to fight a swarm of lickers. Nobody has fed them in a long time.
Chapter 5-1. The entire level has you going around a science facility, and all you ever actually see is 2 lickers initially until the end. You walk around and, horrified, you see the god damn lickerscrawling all over the walls like flies; there's over 2 dozen of them in 2 glass rooms. Any loud noises sets them off, so you quietly head to the next door thinking 'oh God oh God oh God oh God', then you have to break down a door noisily and haul ass to the elevator. And the elevator takes so long that the first few rows of lickers will be at the door before it arrives, so you have to shoot them. The 2 glass rooms actually spawn many more lickers than you can see; namely, there's well over 40, and on the hardest difficulty, they will kill you with 1-2 tongue pulls.
Good news about the lickers: they are no longer poor sods reanimated twice by the t-virus. Bad news about the lickers: they can now reproduce. The Squick-ness of it is horrifying because they are essentially reanimated bloodied corpses humping, to create new bloodied corpses to hump YOU.
Chapter 3-1. Swamp/marshland setting and you have to go to a few islands to grab parts of a key. One of them has crocodiles slowly swimming at you and leaping out to 1 shot you. One has you going along quietly when a swarm of Las Plagas infected locals jump out of the water and kick your ass.
Perhaps the scariest thing about the crocodiles is that they aren't even mutated - in fact, they're actually a pretty realistic depiction of Nile crocodiles. They really can grow to be that huge, and it's estimated that they kill hundreds of people in Africa each year. In a series full of terrifying fictional abominations, this horror is very much grounded in reality.
The Duvalia. Forget doing enough damage and being rewarded with a tentacle in your face - try doing enough damage only for your opponent's whole upper body to be replaced by Audrey II. Complete with biting you in half, the upper of which is probably swallowed, if you get too close. And the actual Venus Flytrap part of it is shelled. The part of it you have to shoot to damage it is the tail hanging off the back. Which is actually the remains of the host's ribcage.
Resident Evil 5 also has the Reapers. First response to them is "Oh, a giant insect thing, I can still kill it oh damn it's taking a lot of bullets what's that cloud of smoke it's spraying oh shit it's rearing up run awOHGODWHATTHECHRISTITTOREMYSTOMACHOUTINONEHIT!"
When you get hit by them there's a totally unexpected and hitherto unused first-person view of the creature looming over you when it grabs you, scaring the ever-loving crap out of you, especially if you're not a big fan of bugs.
They aren't even an engineered creature. This is what you get when Uroboros comes into accidental contact with a cockroach.
The Lost In Nightmares DLC. You go through the first part of the mansion expecting anything to pop out at any time.
On your second playthrough, the Blob CAN sometimes appear in the first part of the Spencer Estate. The good news is, he only pops up on Professional mode. The bad news is he will chase you through the house.
What's REALLY terrifying about the Blobs is the kill screen, should you prove an unlucky-enough bastard to become a squelchy sound effect and the horrifying realization that those are Chris' limbs hanging limply from the top of the screen, where the anchor has impaled him through the stomach and he's most likely still (however barely) alive. Go on, try it. Sweet dreams.
Simply put, Lost In Nightmares makes copious use of Nothing Is Scarier in its first section, with many bonus scares for those who played the first Resident Evil. The L-shaped hallway is back. When you walk through it the first time, you hear dogs barking outside. When you walk back the other way, the windows break, giving a big ass Jump Scare to all those afraid of a Cerberus popping in, only for it to turn out that it was a mere bat breaking through the window. And all the strange stuff that happens, such as hearing footsteps from somewhere in the mansion, or the piano being played. Particularly creepy about the piano part is the fact that when you get to the piano room to check out who's playing it, it is empty - and the only exit is the door you came in through. Once you get into the dungeons, you meet the Guardians of Insanity, complete with short glimpses of them walking past on balconies or through cell doors before you actually do meet them. They have a REALLY creepy theme, and the first one can spawn right around a corner, swinging its hammer down onto you, or on the stairs you just walked down, even when you searched the entire upper floor and absolutely ensured that it was empty.
What happened to Jill. She's been captured by her worst enemy, used in experiments, forced to infect people, and forced to fight her old partner and his coworker. And she was completely aware of what she was doing when she fought against them.
Die in the first level of the game and you are treated to a first person view of the Majini brutally hacking you to bits. Now you know what the victims of Mohammed Farah Aidid went through.
The tough Giant Majini you occasionally battle in the Wetlands and Ancient Ruins. The faint ululations you often hear, moments before you catch a glimpse of one in the distance and the battle music starts... Not to mention the gigantic odd-looking masks that mark them out as different, the clubs made out of human skulls they wield, and the fact that they are about 7 feet tall. The fact that they mostly walk calmly towards you and make very few noises when fighting doesn't help either.
Dear God, Irving's transformation. Becoming a sea monster is bad enough, but being the tongue of said sea monster is just horrifying.
In the first level, Chris says to Kirk that the gate they need to get through is sealed; a few moments later, the Executioner joins the battle and knocks it down effortlessly. And he doesn't stop coming after you unless you kill him. And let's not forget he has what looks like jagged nails protruding out from all over his body, takes several hits to beat and can very quickly put you into Dying status. Have fun.
Uroborous is a virus that causes its victims to be covered in black slithering tentacles that literally eat the victim from the inside. It gets worse when the Uroborus virus infects someone; the person is left in agonizing pain until they are killed or absorbed by the virus. And Wesker was trying to spread this.
There's one area near the end of the game, that has a group of corpses being disposed of in a incinerator. The creepy thing is, if you get even close to those corpses, they will get stand up and attack you while others are awoken — for the first time since Code: Veronica the game throws zombies at you and it's a frightening shock.
The swamp level. The infected Ndipaya tribe are horrifying to behold, their lips and hands eaten away, their hearts bulging out of their chests, and the look of sheer hatred on their faces. They charge screaming at you with inhuman agility and stamina. Then there are the crocodiles lurking in the water to eat you when you least expect it.
The implications of what would happen if Wesker's plan had succeeded are truly staggering. Umbrella's research determined that one in every 10,000 people would have the potential to become Tyrants or gain Tyrant-like powers, meaning that people within that threshold of genetic potential would survive Wesker's global saturation of Ouroboros. 1 in 10,000 people in a population of 6 billion (the population at the time of Resident Evil 5's release) would be 600,000 survivors note 1 for every 10,000, 10 for every 100,000, 100 for every million, 1000 for every 10 million, 10,000 for every 100 million and 100,000 for every billion, hence 600,000 people meaning that 5,999,400,000, people would die (unless you count being a zombie as "alive") as a result of Wesker's plan. The implication of 600,000 people with powers akin to Wesker is bad enough, but having those people running around in a world where society has collapsed, as a result of the sheer devastation brought on by the mass number of dead and infected invokes imagery of hell on Earth if there ever was one. Things only get worse when you remember that total societal collapse on the scale would remove all meaningful infrastructure that allows for comfortable living, leaving this new race of super humans to form clans and live off the land to survive; all the while fending off the vast numbers of undead that surround them. The survival of the human race in such a scenario would be extremely bleak, if not a direct path to extinction outright. note Though it should be noted, while 600,000 people is a rather bleak population size, humanity has reached numbers as low as 10,000 people before, and the general consensus is that you'd need a population of about 1000 genetically distinct people to repopulate the human race with no in-breeding. Also, since these super humans would have an advanced genome augmented by the virus they'd be more fit to survive harsh environmental factors than their ancestors. While harsh, Wesker's successors would have a higher chance of continuing the human race than previous near-extinction events humanity has faced.
The Uroboros virus doesn't destroy or kill its victims, it turns them into mindless, bloodthirsty tentacle monsters ready to absorb you. And looking at Excella, the biomatter doesn't need to be alive for it to grow into something truly monstrous. So imagine 600,000 people surviving with superpowers—maybe some of them children, who knows?—amidst 5,999,400,000 tentacle monsters, that only need to happen across a lone cemetary to become something big enough to destroy a major city and even drink up some ocean - with whalefalls as afters. Just imagine them all converging together until they've consumed all life on the planet, and if you do, a Moon-sized living thing that's also a vast mass of tentacles is unimaginably horrific to even think about.
Thankfully, Wesker foresaw this and had TRICELL launch The Shango Laser Satellite as a countermeasure against massive Uroboros creatures.