This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Luigi's Mansion: Dark Moon
King Boo actually looks rather frightening this time around, especially when he appears behind Luigi from the darkness at one point.
He also broke out of his painting. On his own. Before getting the new crown.
The final security image. Whoo boy, that picture. Throughout the game Toads give visual status updates, and gradually a menacing figure becomes apparent on them. An unearthly glow just off-camera, a shadow whose owner cannot be seen, and so forth. On the final image, King Boo is not even trying to hide himself, floating in plain view of the image. And when you examine him, he turns towards the camera and cackles maniacally.He'sknownyou've been watching him all along.Because as you soon find out,he'sbeen watching you. The absolute worst part? This happens on a still image that is otherwise devoid of animation. He is that damn powerful. To top it off, as he laughs, the screen breaks off into static before shutting down, ala Slenderman and every other horrible "caught-on-camera" abomination. Long story short, Bring My Brown Pants, gents.
To top it off, in most of the scenes when you're looking at the latest photo, you look at it to an arrangement of the first Luigi's Mansion theme. When you look at the final security photo, there is no music at all.
Let's look at his motivations from the last game. He stated he wanted retribution on the Mario Bros. and Gadd for the trouble they caused him and his kin in the past. Working on behalf of his kind; understandable. Here? He deliberately shattered the Dark Moon to break the minds of the ghosts residing there to experiment on and enslave them into an army to conquer the world. Yikes. Combine that with how his eyes seem even more sunken in than before and how he obsessively stalks Luigi towards the game's end? Call it crazy, but King Boo might be a tad unhinged after his last humiliation.
King Boo's design has drastically changed since last time, too. Before he was just a giant Boo wearing a crown. Here... His eyes are purple and sunken in. Unlike other Boos, he wears a constant toothy Slasher Smile and the inside of his mouth has turned blue, unlike the red it was before. It really looks like he's gone off the deep end.
Even worse is that King Boo states that he plans a similar fate for not only Luigi but all of his friends as well.
The fish tank in Gloomy Manor contains an angelfish with a giant toothy maw where its face should be.
In the E-1 mission, look in the window on the right immediately after approaching the door and having Polterpup appear. It's not only scary, it's just plain wrong. You actually see a Toad in the process of being trapped in a painting by a couple of Boos, and he's downright terrified. (On the plus side, the other Toad's painting is seen in the background, so this gives foreshadowing for the E-2 mission by letting you know that there are two Toads beforehand.)
The playroom in Haunted Towers is very creepy, at least compared to the other, less frightening rooms. There's a doll house that mindscrews you if you look into its window: you see the exact same room you're standing in, with Luigi looking through the same window. Then there's the obligatory creepy doll whose head rotates 360 degrees and the loud Jack-in-the-boxes. And the room is pitch black at first.
The boss of the ice mansion when he possesses the ice itself, thanks to its spooky face, introduction and its evil laugh. Probably one of the more intimidating-looking bosses in the game.
The cutscene right before Luigi enters the Mansion in Mission E2. Luigi opens the door, but finds only darkness... as he turns around and scratches his head, King Boo appears right behind him with a horrible, evil smile, but vanishes before Luigi turns around. If that doesn't scare you, nothing will.
It's even worse when you know how he appears. Luigi opens the doors and activates the flashlight to look into the darkness. He then turns around, understandably confused. There's a brief moment of lens flare from the flashlight, and in that one second, King Boo appears. Floating perfectly still with a mad smile, and he vanishes out of sight just before Luigi turns back around.
And arguably the worst part is, it can't be intended for Luigi. After all, the plumber never actually sees it; by the time he turns around, the lights are on. King Boo isn't trying to scare Luigi here; he's trying to scareyou.
The cutscene before the final boss fight. Just imagine how frightening that whole scenario was for poor Luigi. Without warning, he's suddenly been transported to an unfamiliar area where it's pitch black. Then, windows begin appearing all around him, and an all-too-familiar laugh starts coming from... well, everywhere. Then, as Luigi spots the painting Mario's trapped in and starts to reach out for it, King Boo appears right in his face. And the worst part of all? King Boo is pissed. No wonder Luigi was shaking like a leaf in there.
Even worse, you can tell the reason King Boo doesn't just end Luigi's game in the dark: He's screwing with Luigi. King Boo wants Luigi to die alone and frightened, and it's terrifying.
In the second mission of the Secret Mine (D-2) Luigi discovers a room full of Boos, along with a Greenie, a Slammer, and a Sneaker trapped inside red crystals that the Boos have put them in. It becomes disturbing when you realize that the Greenie, Slammer, and Sneaker are all terrified and are banging against the sides of the crystals, voicelessly pleading to be released from their containment.
The music for the garage sounds right out of a horror game.
The Spider boss and the cutscene after it. After the ghost possessing it is defeated, the spider returns to normal and goes home. Luigi turns around to find a stampede of baby spiders following her. It's mostly Played for Laughs, but to an arachnophobic, it's a nightmare come true.
Sometimes, when you're being Pixelated somewhere, there's pixels in the stream that are a different size and color then the ones that Luigi turns into, and they're traveling at a different speed than the Luigi pixels. Someone or something else is in the stream with you.
It's most likely King Boo himself, who abducts you in the middle of your return journey from the fifth boss mission (E-Boss).
Treacherous Mansion itself is rather creepy, especially as the game comes to a close. All alone at the furthest corner of the valley, smack in the middle of an endless ravine. And inside the mansion is a pitch black maze chock full of enraged, berserk ghosts and King Boo. And it just gets scarier as it goes. First, we Shoo Out the Clowns by finally capturing Polterpup for good, and rescuing the last of the Toads. Not scary, but it puts an end to any lightheartedness. From there, it steadily gets creepier until the fake boss of the mansion.
Big Boo's battle has a creepy song playing, like the carousel in Big Boo's Haunt. And the context of the fight King Booknows what you've been doing. He anticipated you reaching the toy train tracks, so he stationed Big Boo there as a trap. The game's objective even says "Escape the trap", not "beat the enemy", making this feel even more like a situation you shouldn't be in.
And it doesn't end there. For the next mission, the mansion is completely empty (save for one Boo you can catch). It's just you and King Boo. Nothing Is Scarier in full effect, the music is only helping the mood. You walk in on King Boo at the terrace, just as he's opening a demonic portal. If only you'd been able to avoid Big Boo's ambush. What follows may qualify as That One Level, but the game's stakes are at an all-time high, not unlike Super Paper Mario: King Boo unleashes an entire army of berserk ghosts in the mansion. If Luigi doesn't capture them all within the time limit, their combined paranormal energy will cause Luigi's dimension to collapse on itself. Just like that, King Boo is able to match Count Bleck, and at a much faster rate. He'snotmessing around.
A more gradual and paranoia-inducing factor is how the mansions change from mission to mission.
Gloomy Manor gets overrun with spiderwebs. Finding one blocking a path that was free until the previous mission is quite jarring, especially to aracnophobics.
Haunted Towers get enveloped by overgrowth, with plants occasionally breaking through brick and mortar and generally becoming more and more commonplace.
Old Clockworks gives some respite, but pretty soon you get to meet empowered ghosts for the first time.
Secret Mine takes the cake: not only it gets darker and colder (a snowstorm starts, places with water freeze, lights get dimmer as torches are blown out) but powerful ghosts become more frequent than their normal counterparts, especially once you see how they're created.
Treacherous Mansion has rooms getting more and more overrun with ghosts, and harmless furniture turns into traps in later missions.