Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Soul Eater
As expected of a series that takes place in a town called Death City, Soul Eater certainly has its fair share of terrifying moments.
Practically all the Nightmare Fuel from the anime is in this video.
His name is Soul Eater. And he does. With great pleasure. And nobody sees anything wrong with it.
As for him, so for the other Weapons. The manga more-so than the anime mentions that, while there are rules about which souls can be eaten, the origin of these rules—Lord Death—makes the whole business arguably suspect depending on how you view the Dumbledore-ish Eldritch Abomination Grim Reaper. Mifune, for example, was said to be a legitimate target despite his peculiar circumstances (he only fought to protect Angela, a witch too young to be a threat to Shibusen or anyone else).
At least this is the case in the manga. In the anime, Mifune is never suggested as being a target of soul collection. It's more an issue of the fact that (as of the Arachne arc) he works for the villain, so they were allowed to kill him if necessary, but taking his soul was off limits. In the manga, though, Lord Death totally suggests that Black☆Star should take Mifune's soul, as well as Angela's, right from the very beginning. (Though, to be fair, it's implied that he only suggested it because he knew Black☆Star wouldn't do it.)
According to Soul Eater Not!, they're learning to do all of this at what the world sees as a school for superheroes! It's a world that worships death, and the resident super hero academy is teaching kids to kill people. Either as a meister using the weapon, or as the weapon who eats the murderer's soul. Wow.
Just the thought of what the FUCK happened to Crona and Ragnarok to get Ragnarok melted with the black blood, and then replace ALL of Crona's original blood with that black blood. Not only that, but what about the effects? Because of the Black Blood, Crona would be literally 'cold-blooded'. He/She wouldn't give off that same human warmth that most people would. It would basically make Crona inhuman. It's no wonder Spirit was so pissed at Medusa for what she did.
Earth had been depicted as a weird hodgepodge of times and places. But after being defeated by the good guys, Medusa, in the form of a snake flees to a perfectly normal suburban neighborhood, indistinguishable to the Real World. She takes over the body of a little girl who was with her mother by crawling down her throat. The mother and father know something's wrong with their little girl, but they don't know what. And then she disappears, and the scene shifts to signs of the little girl missing. And those signs are old, so we know she's been missing for a long time.
And Medusa speaks in such a Creepy Monotone when she's trying to pass herself off as the little girl to the girl's parents!
What's the only way to make Medusa's creepy-as-hell Slasher Smiles even worse? Put them on a little girl's face!
The Clown is insanity incarnate. And its mismatched eyes and stare will haunt you◊.
In the manga, Crona's Magnificent Bastard mother Medusa forced him/her to KILL A RABBIT IN ONE FLASHBACK. Back to the present, he/she's leaning over Maka, holding Ragnarok over his/her head, wearing his/her scariest face yet, Maka's head replaced by that of a rabbit.
Oh, it gets worse. After Crona kills it, Medusa says something along the lines of, "We'll have rabbit for dinner tonight." That's right, she didn't just make him kill the rabbit, she made him eat it too.
Additionally, the Demon God Asura leaves quite an impression.
A mere hallucination of Asura rips the faces off of Free and Eruka with its teeth and then just disappears. Then later his actual body emerges, slowly, painfully, creeping across the floor towards the protagonists in a way that is completely inhuman. Oh yeah, and this is after Shinigami flayed him alive and put him in a bag made of his own skin. And when the hallucination ends, Eruka is seen holding a sharpened spike at her throat, and Free has a large ass blade half way down his neck.
We then get to seem him, hollow-eyed and emaciated, putting his skin back on from the inside out. He then proceeds to stretch it like elastic to cover himself.
Then there's his actual face◊, where there are eyes within his eyes. Looks rather cool, but jeesh.
Just the sheer Mind Rape that Asura's madness can cause is the stuff of nightmares. There's no way to know when it'll strike, you can't stop it, it infects you and destroys your personality until there is nothing left, making you a danger to those around you and sometimes driving you to suicide. The music in those scenes, namely "Victor", does not make things better.
Tezca Tlipoca, the weapon of the South American Death Scythe, a monkey named Enrique. You'd think he'd be the meister as a human, but you're almost grateful he isn't when you consider the fact that he's a guy wearing a bear mask (reminiscent of Hyde and Closer) who can trap people inside horrible mirror illusions,
No 'love' for the Book of Eibon? The thing has teeth, is pulled out of nowhere by Noah, and Kid - who up until this arc has not been fazed by anything very much (asymmetry excluded) - is screaming as he gets dragged into it.
And then there's the fight between Medusa and Stein and Spirit. Or, more specifically for this entry, the way Stein acts during the fight. Stein's shrieking and lashing out with Death Scythe is both awesome and horrible, and the grinning doesn't help.
Some of the anime music is a mix between Nightmare Fuel and Paranoia Fuel. "Apparition" and "Like a Radio" in particular come close to stealing your sanity. Oh, and comparing "so scandalous" from the first OST to "Soul Eater - Reprise" from the second makes you want to beg Soul to please please please stop using the Black Blood, now.
Giriko's actions towards Maka are extremely disturbing, as he makes it very obvious he intends to rape and kill her. Oh, and we get a handy reminder/guess from Giriko she's 14. Great.
The manga has the Clown chapters where Maka is under a hallucination which starts out as a harmless-seeming flashback, filling in a couple of bits about her past. And then everyone falls down dead. Amidst the other Body Horror and Madness tropes, my worst/best bit of nightmare fuel in this series is seeing a little Maka trying to get her daddy to wake up.
Chapter 22: Medusa's armless torso crawling up onto Stein's back and attempting to bite his neck! Made even creepier when she whispers "I love you~" before he stabs her through the face with Death Scythe. Which we see in a nice big panel. She is still grinning with the blade sticking out of her skull. The anime doesn't show this. If anything, Okubo's simpler style early on only makes what Stein does to Medusa all the more horrifying. Ditto with Crona's mad faces and the 'rabbit' mentioned above.
On the other hand, when Okubo does decide to depict bloody injury, we get stuff like Mosquito beating up Kid, leaving a trail of blood from his missing arm as he throws the boy across the floor. Ow. Oh, and Patti's still in the hand which is lying "all the way over there". Mister Okubo then goes to town with Mifune and Black Star...
The Flying Dutchman getting brutalized by Crona. When he rows away (That's right, he lived!) from the destroyed Nidhogg, his face is laying on the floor in two halves, so you get to see his TONGUE coming out of the top of what's left.
Death the Kid becoming an Omnicidal Maniac in the manga. Sure, he gets better thanks to Black Star. But, still, the very fact that he was perfectly willing to destroy everything (including his own father and his own weapons/partners) just to achieve perfect symmetry through nothingness is terrifying. "My skin is crawling...."
An extra bit of Fridge Horror accompanies Kid's temporary insanity as of chapter 107. Asura is revealed to have been created similarly to Kid from a fragment of Shinigami's "terror". Kid's descent into insanity focused solely on order because he was a fragment of Shinigami's obession with law. The same thing happened to Asura, except that there seemed to be no one willing or able to bring him back to his senses and now it is almost certainly too late.
Justin Law in chapter 86.◊ The madness version of his 'blade of faith' speech from his debut is also a nice touch and suitably horrible. Anything Justin after he starts suffering from madness can be nightmare-inducing.
Why does he start suffering from madness? Well, the insanity clown that nearly took Maka complained that it didn't work on her, like it did someone else... Justin spends most of the series under More than Mind Control and, instead of Tezca managing to save him, kills at least two people he'd normally consider friends and dies fighting against everything he once believed in. And I Must Scream, already, but does anyone ever figure out what really happened to him in-story? Not on-screen, anyway.
Crona going berserk and killing Medusa. It should be a Crowning Moment Of Awesome (don't lie, we all know the old witch didn't deserve to live), but the way it happens (Crona ranting about Medusa forcing hir to give up Maka, proceeding to hack Medusa to pieces with a completely blank expression and Medusa's massive Slasher Smile and the revelation that this was exactly what she wanted) makes it horrifying.
The entire sequence starting here. Crossing over with Tear Jerker and Adult Fear, it is perhaps the most horrifically realistic example of child abuse in the series. Simply put, Medusa gives Crona a Sadistic Choice; s/he either kills three bunnies in three different ways or starves. Crona refuses to kill the bunnies, so Medusa locks hir in a room and refuses to give hir food for five days. When Crona is finally let out, s/he is given the choice again. If s/he kills the bunnies s/he can eat. If s/he doesn't?
Not as bad as the other examples on the page, but creepy nonetheless: The fourth ending to the anime, as well as having some apocalyptic opening images, has a long pan of all the secondary characters lying dead amongst wreckage and decay. Combined with the much darker tone of the music, it makes for an intense ninety seconds.
While we all know it will (eventually) turn out okay, the scene with the secondary characters is rather similar to the one following Asura's last attack on Death. The explosion leaves the onlookers lying unconscious in the rubble, and Shinigami in pieces, with Spirit having been thrown to the ground elsewhere.
All of the new clowns in chapter 91, especially the White Bunny (in the link directly above). Because one clown wasn't enough, they had to get more disturbing.
The fact that, in order to put Ragnarok into Crona's body, Medusa literally had to melt him down like he was an inanimate piece of metal, then mixed him with the black blood and injected him into Crona (who was drained of all his/her own blood). And this while he was apparently a child. Exactly how terrifying and painful must it have been to go through all of that? It's surprising Ragnarok is just a Jerkass and not a mindless, bloodthirsty wretch.
It may be just a reflection of Crona's own deteriorating mental state, but this is pretty much what he seems to have become in the later chapters of the manga, only uttering strange noises while his head pops out of Crona's midsection every so often.
If this is so, then It Gets Worse because Ragnarok is not seen at all later on, not when Maka and co. confront Crona in Italy, nor when they end up fighting. Which means that when, IIRC, Stein and Spirit expressed concern that Ragnarok would consume Cronas soul, it seems to have been the other way around. Crona has control of the Black/Mad blood, with apparently nothing of the Weapon's consciousness remaining. A meister consuming a Weapon's soul ties in both to the original tale Spirit told Maka about the Kishin, and into what might have happened with Asura and his vajra (of course, hopefully, it may not be a Demon Weapon at all...).
Professor Stein goes completely insane as a result of Medusa's plans. He got better, but still the entire collection of scenes showing his descent into madness is creepy.
Pretty much anytime anyone in the cast undergoes Sanity Slippage is this. Which is often.
The battle on the moon is an exceptionally weird example; with Stein's almost pretty hallucinations ('unwrapping presents'?) being revealed as a blood-stained battlefield, with Spirit looking on in horror.
Black Star's arm being mangled almost beyond recognition by Asura's attacks.
Chapter 112: the very end where Asura claims that he will return and tells the characters/reader to wait in fear until then.
During the battle on the moon, Stein reflected on what he was doing and why. He concluded that he won't give in to the 'Madness' because Spirit, Marie, and the others were with him. As of ch112 Spirit and Marie may be dead. Now we'll find out whether they were ultimately Morality Pets who led Stein to appreciate the people around him (as more than test subjects) or Morality Chains who were holding him back. This troper's money is on the former, but it's a disturbing thought.
Stein referring to his own child as a guinea pig. Is he really any better than Medusa?
He does have someone (Marie) to stop him from doing that, something Medusa didn't have. Still pretty unsettling (and funny, in a twisted sort of way), though.
While thematically the Power of Friendship/Love/Morality Pet thing fits, and has been shown to work for Stein, there's still the question of how far it works. What if something happens to Marie? What if something happens to their child, because of Stein? Too much could go wrong. That being said, one of the general messages at the end of the series seems to have been that, where humans are concerned, a lot could go right as well.