This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Child of the Storm
The entrance of the Winter Soldier. When he first appears, it's left unclear exactly what he is, and has a room full of hardened Death Eaters and Werewolves terrified of him. Without saying a word. The only people who aren't scared of him are those who know he's coming and Gravemoss (who's completely Ax-Crazy anyway). Though, the heads of the manticore and the nundu rather helped make an impression.
The backstory of Hela, crossing over with Tear Jerker. Loki seemed to be experiencing an irrational case of Adult Fear about his lover and unborn daughter and his parents (father's) anger, so hid them away. However, the birth went wrong and both mother and child died. Loki used necromancy for the first and last time to resurrect the child, Hela. And then she became something of a Creepy Child and had an involuntary case of Vampiric Draining - small plants and animals tended to die around her - required to keep her in the living world. So Loki, with the help of Thor, Sif and the Warriors Three (apparently how they met each other), raises her in Helheim, but has to leave her there eventually.
From Frigga's point of view: her younger son has just disappeared with the pregnant girlfriend that, of course, she knew about, she knows how dangerous a birth can be so she's desperately looking for him, and when Heimdall finally picks him out, rushes to the scene, as fast as she can... to find her son, his arms bloody to the elbow, weeping, with her firstborn grandchild dead in his arms while his beloved girlfriend is already dead. In essence, she was too late.
And the experience drives him to use necromancy. Dark magic. As a powerful sorceress herself, one who taught Loki the ways of magic, that must really suck.
From Odin's point of view: he essentially set the whole thing in motion by being such a distant, frightening father that Loki would rather run and risk who knows what then own up and seek help. It is all on his head. And you can bet that he knows it.
Loki's time with Thanos was this, according to what little we've picked up from him.
The Darkhold doesn't just grant the means to create veidrdraugar. It grants the means to put them into mass production.
Gravemoss rips Sif's heart out. With every intention of turning her into one of the veidrdraugar. While Dresden manages to prevent this and Doctor Strange pulls a Deus ex Machina, but it comes very, very close...
In chapter 60, after winning something of a Curbstomp Battle against HYDRA's werewolvesthe aged up kids seem to be coasting to another victory with minimal actual peril faced... then, in a matter of seconds, they are curbstomped by a robot based on the Destroyer controlled by Baron Zemo which inflicts some of the first genuine, non-trivial injuries on them and despite the fact that Harry and Diana are in Thor's league of power, comes very, very close to killing several of them. This isn't the conflicted Wouldn't Hurt a Child Winter Soldier or a less powerful Elite Mook. This is someone who not only wants to kill them but very definitely can and wouldn't think twice about pulling the trigger.
Chapter 65 exploits the Fridge Horror of Ginny's experience at the hands of Riddle's diary, having Sean explain to the Hogwarts staff that Ginny has considerable psychic trauma following her encounter with Tom Riddle's diary. When Snape questions this, Sean retorts:
Being a man who knows what he's talking about, he then goes on to directly compare Riddle's diary to a sexual predator, explaining the parallels. Afterwards, the staff look a mixture between sickened, horrified and homicidal.
Chapter 66 presents the scenario of being trapped in a memory of a violent superpowered battle powerful enough to shatter mountains with a young psychic who's completely lost control of his powers which are making everything real.
Part 2 of Chaos Reigns gives us a glimpse of the horrors that are breaking through into Earth due to the Darkhold being free, as an army of N'Garai and Mindless Ones, led by a Mabdhara, swarms into Chicago, leaving a trail of death in its wake. The scene of one their feeding spots is enough to make even Dresden, who's used to this sort of thing, retch. And he notes that, as long as the Darkhold is still loose, this is just going to keep happening...
Chapter 71's description of HYDRA unleashing the veidrdraugar on their targets and when that (rarely) fails, they grab people off the street, put them under the Imperius curse and inject them with particularly unstable Extremis, turning them into unkillable living bombs, is fairly horrific.
Chapter 72 reveals that Luna died from a bullet in the lung. This means that she would have died slowly and painfully, drowning in her own blood.
It also has Thor living every parent's nightmare: finding out that his son has died. The fact that he was resurrected immediately afterwards does not make it any better.
Chapter 74 has Harry finally snapping and demonstrating just how mindbogglingly terrifying an enraged psychic can be.
Chapter 77: Chthon rises, which turns the skies red and unleashes demons over the entire Earth.
Hell, pretty much every aspect of Chthon's appearance is designed to show that this is just Wrong, capital letter fully deserved.