This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Devil May Cry
Devil May Cry 1
The entirety of Mallet Island gives off this unrelenting vibe of a nightmarish location when you examine the libraries and documents early on in the game. Crops and plantations were irregularly reaped, with Winter crops coming in the Summer and vice-versa, a book detailing how, at night, the statue of Mundus at the main hall would suddenly vanish, and not to mention the alarming number of executions and incarcerations that took place at the castle itself. There's even a library book Dante can examine that says the castle grounds went through excessive renovations and reformed, with the author even asking if the builders and residents were possessed by evil spirits.
The moment night falls and you come back out to the open courtyard, both before and after heading into the Colosseum. The previous ambient music and wind sounds are gone, replaced by the sounds of crickets... And several animalistic growls and noises. Stock sounds, surely, but in particular comes a noise that sounds close to that of a monkey... Which sounds eerily like evil laughter. And since the island doesn't show any signs of life except for Dante himself and the demons, are those truly animals you're hearing?
The castle at night. The layouts of several rooms have changed and everything is now dark and dead silent, save for that one music that continuously plays in the background, and the occasional presence of a demon. Aside from that and the bosses you fight, the entire place is now openly hostile against you, as opposed to only overrun by the enemies.
Reading the castellan's memo in his bedroom in Mission 4. He speaks of how at some parts of the castle, flowers take a lot longer to bloom while in others, they wilt as quickly as they sprout. The island wasn't just being invaded by demons, it was probably already theirs.
Fridge Horror settles in when you realize that, at some point, this island was filled with humans keeping the place up and running... By the time Dante arrives, any traces of human activity are just gone, not even bloodstains or bodies are left behind except for maybe one or two, outside of the castle at best. Naturally, the demons very likely killed everyone on the land, but what then? Given how demons in fiction tend to act, well...
Nobodies. Freaky humanoid...things that crawl on all fours, throw EXPLODING EYEBALLS at Dante, all the while laughing and howling like monkeys. And if one dons a mask, the Nobody grows twice its size and can siphon Dante's magic (Devil Trigger) by...some kind of ritual dancing. Not to mention you first encounter them in the Mirrored Castle, which is saturated with red, with the camera tilted slightly and the screen mostly blurry and surreal. If Dante gets grabbed by a Giant Nobody when his health is critical, the enemy snaps Dante's neck. Complete with a clearly audible *crack!* sound.
The Mirrored Castle itself is pretty damn freaky, with the odd camera angles, the blurry look to everything, and that damn music.
As early as Mission 4, once you can enter the castellan's bedroom, which houses some locked doors and the mirror from which Nelo Angelo emerges, checking said mirror will prompt Dante to comment that while the mirror is rather beautiful, he can sense that something is just off about it. Sure enough, many missions later, Dante enters the mirror and is proven right. On the way back to the door to the mirrored bedroom, off in the distance is an enormous, howling vortex (part of the Gate to the Underworld). Checking it will have Dante say that while it's too far away for him to reach from where he is now, he can sense a massive amount of evil energy pouring out from it.
Even the normal Castle music is unnerving with the goth design adding to the soundtrack as you explore the empty rooms...well empty at first.
Even the early enemies in the game are quite creepy. Marionettes are giant puppets possessed by evil spirits, wielding sharp hand-blades or shotguns. Bloody Mari have their clothing dyed with human blood. Sin Scissors have the ability to phase out of walls and pictures and have a really disturbing laugh. If you read the manual, it says that the Sin Scissors like to decapitate humans while they're still alive.
What's even scarier is some the original concept art for enemies that never made it in to the final game (which you can find in the vault for the HD collection). The prototype designs for the demons look more like lab experiments gone wrong from Resident Evil. The Blades look like demonic Hunter βs', and the concept art for the Shadow looks like a rotting, decayed, panther/tiger hybrid. Even the concept work for the Marionettes are creepy. One artwork has the marionettes look more like dried out, rotting corpses, all hanging from nooses on the ceiling; implying that the marionettes were not puppets made of wood possessed by demon spirits, but were once humans now possessed by demons.
This is actually a justified nightmare fuel in a production sense. Production-wise, DMC was essentially a prototype for what would later become Resident Evil 4, so it makes a certain degree of sense that the original concepts resemble more of the Umbrella Corporation's disgusting creations.
And you don't even kill him. The best Dante can do is (much like his father) seal him away with Trish's help. Dante may have been joking when he told Mundus to give his regards to his son, but what if Mundus returns?
Nightmare. It pretty much exemplifies the idea of an Eldritch Abomination, as the enemy file says it's unclear whether or not the thing is even alive or just some repulsive weapon. In addition to being incredibly difficult to fight, it's freaky as hell, looking like something right out of a Giger painting. It is capable of sudden rapid movements, including shooting giant spear-like appendages out of its body at you. In its normal state, it is a sickly mound of goo, filled with half digested corpses of its previous victims, and it constantly tries to trap you in a literal nightmare world.
Read the file on Nightmare's "trap" move. The world that Dante/the player is sucked into is "a representation of the trauma that rests deep within Dante's subconscious" — the loss of his beloved mother and his brother.
A late-game mission takes place in the Underworld. It's a sickeningly creepy Womb Level, complete with life-sucking tendrils coming out of the walls, pulsating tissue walls you have to hack through, and visibly beating "organs". During the timed final mission of the game, if you let the countdown get to zero, the screen fades to white, an explosion sounds, and you get treated to Dante's bloodcurdling death scream.
Phantom and the Kyklops are not a good sight for arachnophobes. Especially since the former will chase you in the tight corridors of the castle, spitting lasers at you and bellowing. Phantom's fatality? He swallows Dante whole.
The first game is the only one with fatalities (Dante's vitality meter must be completely red for these to happen), all of them horrifying to watch. As mentioned above, Phantom will grab and devour Dante alive; Giant Nobodies will snap his neck. Sin Scythes will impale Dante, swing him around like a flag before tossing him to the ground. Shadows in their red state will swallow Dante, bash him around and explode, leaving no trace of him left. Blades will dogpile Dante and slash and beat him to death. Death Scythe will throw Dante into the air, and then use his scythes to cut him to ribbons offscreen. Even the most common Marionette enemy will latch onto Dante and gash at his head and neck even after he falls down dead.
Devil May Cry 2
The second game has its share of bad things, but Nefasturris and Nefascapitis are among the freakiest. First of all, it comes to life out of a skyscraper and is a gargantuan devil composed of a whole mass of small bat-like vermin and stares at you with a clear eyeglobe. When you defeat it the first time, you have to fight its head... Which stands on its own with its lower jaw split open.
In the final level, the influence of the Demon World has spilled out and warped Dumary Island into something fierce and disturbing. The town is now covered in webs and weird-looking cocoons, the waterways are blocked by the same webbing and even the buildings themselves look odd and twisted, some of them at weird angles. The creepy music doesn't help.
Arius' building near the endgame, where both Dante and Lucia must ascend to the top floor. As you proceed, you find key items that twist the area even further... And then that three-faced demon shows up.
His name is Trismagia, and yes, he is freaky. His regular appearance is that of a gigantic, three-faced head with four eyes. As the fight begins, he splits into three individual faces. However, if you look closely, you'll notice that he can't separate his body as easily as he fuses. He instead painfully writhes around for a few seconds before forcibly ripping himself apart, with blood splashing into the air. Seeing as Trismagia tends to return to his base form periodically to fire a single, powerful blast at you, you'll be treated to this wonderful visual multiple times. Each of the faces have their own elemental affinity and associated emotion: anger = fire, melancholy = ice, and joy = lightning. Unsurprisingly enough, the lightning face with its Cheshire Cat Grin is the creepiest-looking of the trio. Made even worse when it comes at you, getting quite close to the screen.
Its voice is also rather creepy. Like the sound of someone who's had their throat slit.
Trismagia: The Son of Sparda... You must repent your sins!
Devil May Cry 3
How about the Hell Wraths? Those pulsating bombs they carry that look like giant tumors, that horrible, tortured moaning, the fact that they could explode at any time....
On the same tune, the "Faceball" that Jester summons in his boss fight can spit said bombs at you... And they look like overgrown, freshly pulled out hearts.
Jester in general. Anyone else get the subtext of his treatment of Lady during The Reveal? When she rushes to stop him, he grabs the Kalina Ann, flickers his tongue at her, and says "Don't be a bad girl, Mary, or you can expect a spanking from Daddy later!" Now, that wouldn't be so eerie if it weren't for the evil nature that lies behind all that goofiness. The overall vibe still seems very wrong somehow, considering he's her father.
Many monsters, especially the Arachne (giant, deadly, half-spider-half-woman hybrid with loud screams) or the Soul Eaters, invisible octopus-like demons that will attack you only if you turn your back on them. Also, Blob!Arkham is pretty freaky looking.
The Demon World itself. In the previous two games, it was often depicted as a desolate, hellish landscape at best, and a Womb Level at worst. This version of it, however, is oddly serene. The "main hall" area is a bright white and it has human-like constructions and structures lying around, forming parts of the background. It looks disturbingly as if a church was pulled into Hell and twisted to fit the demons' views.
The overworld theme for the Demon World also fits with its odd dissonance. It kicks off Mission 18 and at first, it sounds rather angelic and heavenly (which serves as a good tonal call-back to Mundus' cathedral-like room in the first game), rather unfitting for the home of the demons... Until you play your way through the Lost Souls Nirvana, where it reaches its nightmarish portion of eerie chanting, whispers and odd instrumentation. And from that point on, the hellish part is the only part of the song that plays in the Demon World until you fight the bosses or enter one specific room.
The room "Chamber of Echoes" in Missions 5 and 6 has an eerie background music which includes laughing children.
Horse-lovers may find Geryon a bit terrifying, especially with those terrifying neighs he makes...
Reading its library entry is both frightening and sad. Geryon used to be a legendary horse ridden by numerous heroes. But, over time it absorbed too much demonic essence/energy...and Geryon became a demon itself.
Faults, horrible monsters that appear out of the ground with a terrible rattling sound. WallMasters, essentially, these things form on the ground beneath Dante to swallow him and take him to a strange semi-dimension to fight some monsters and waste time. They operate and look like worms with eyes all around its mouth and never stop coming, causing you to have to keep track of them as you fight other monsters, severely reducing your combo chances. Their only saving grace? They only show up in one mission.
Unless you're playing against the Savior in Mission 18 on a difficulty level higher than Human (Easy).
Agnus around Nero after his first "Boss battle" (That he doesn't participate in...). The way he is around Nero is creepy in general, with his obsession over Nero's Devil Bringer, and ESPECIALLY when he has Nero pinned against a wall. He states that Nero will be his next subject of experimentation, and aside from his actions seeming WAY too similar to a rapist and his victim, the implication that if Nero hadn't defeated him the first time around he would be nothing more than a lab rat for the insane scientist are way too chilling to ignore.
The premise of the game is paranoid-inducing. A society brainwashed and conditioned by demons without them being aware of it is not something you wanna think about.
You know you've done something right/wrong when the ingredient of a fucking softdrink can make you want to yak up. The ingredient? A giant slug-larvae thing constantly chained to a mixer.
When Limbo transforms, the landscape becomes horribly twisted and just about everything becomes colored, a jarring contrast to the greyed out palette of the real world. It gets worse when you start looking around the place, subliminal messages crawling around.
In Mission 14, you get to confront Mundus' child, and it is by far the most messed up looking thing in the game.
Whereas the Succubus and Bob Barbas provided the players with an awesome (and hilarious) fight, this child is anything but. Not because it was badly designed mind you, but because of how the whole thing plays out. For one, Dante stops trashtalking and instead sounds disturbed for most of the fight. Secondly, there's no indication whatsoever that the child itself is evil beyond being Mundus' unborn child, and possibly just interpret attacking Dante as "play time" as Lilith puts it.
Mission 15 gives us a nice view of Vergil shooting a pregnant Lilith in the stomach, killing the child and then blowing her head off. Granted, Lilith wasn't the most sympathetic character in the game, but damn.