Interestingly, many things that are considered Nightmare Fuel for other people are considered enjoyable by Jokers.
Joker biological fathers are controling and violent, which goes terribly with Jokers. This group also frequently experiences rape while working as a street prostitute.
Jokers with stepfathers experience the above and are raped by the stepfather since childhood.
Stevens, the template of younger-sibling of Jokers. Have to endure what the Jokers go through, often with the Joker protecting them to some degree. They can be rescued, however without outside difference they can be corrupted and becomeNightmare Fuel.
A template attractor for Jokers is being raped, which itself is horrible, but in Sue's case we get to see first hand, while Aegis is there and vulnerable.
Path being captured and tortured andtouched, forcing Kas and Petaal to separate.
Downside. That is an afterlife where people can be tortured as punishment (or just for the hell of it) by people with People Puppet powers and the entire thing is controlled by an absent and distant entity, but actually run by bitter judges that can read your entire history.
Even better we get to see the worst of it through the eyes of Shell. Who had a particularly bad time of it even by Downside standards, being kidnapped, shut in a box, and tortured for something around 25 years, resulting in near-total memory loss, and then wandering aimlessly for another 50 or so (with more intermittent torture) before running into Strat, and then getting a door to Milliways.
The Land of the Dead. Downside at least has a failed attempt at fairness, everyone is punished in this.
The entire world of Materia. Makes Glass want to throw up just by looking at it.
Sarion had to go through one of Bells' worst fears (having her mind constantly read) to save her world. She ended up mostly catatonic before the peal found her.
Etty is kidnapped and raped every night, expecting to potentially end up with her mind destroyed.
Harley accidentally ending up in Gift, subjected to an attempt at Brainwashing for the Greater Good, used as a Living Battery, and isolated from almost everyone (which is particularly bad for a Joker) and finally teleporting into the Sun just to get away from Chelsa and staying there for five years before the peal finally shows to to rescue him. All because the Theory of Narrative Causality demanded he experience a minimum amount of suffering.
Old and New Kystle:
The mage source of powers are the halfblood, people with one Fae parent that have vast magic and always end up insane.
The mage society is basically run by a selfish medieval nobility with magical powers and without concepts of consent. They regularly went through conquests using their magical advantage over non-magical folk.
Said mages, always more thirsty for power, decided it was a good idea to summon it from other dimensions, resulting in Kystle being invaded, nearly two billion killed and about 20 thousand survivors evacuated to another dimension.
The two situations when Vernaia and Path are touched.
In Gemini, the precog predicted massive eruption forces a good part of the west united states to need evacuation, Conduit and Flicker are part of the effors saving millions, still, they go into the middle of the disaster zone saving people amidst the chaos fire, but watch many die as well.
Angband is pure nightmare fuel - you are not only tortured and raped, and not only does this happen in time dilation, but there are hallucinations of being rescued that melt into being back in Angband and contain fake versions of people you know doing horrible things. When people are actually rescued, they don't believe it. And then there's the memory modification.
All Eru Iluvatars are pure nightmare fuel. They have created a possibly infinity of worlds that are designed to be as tragic and painful for its inhabitants as possible. All of the living creatures have everything stacked against them. The scariest thing is that Eru does this purely to get entertainment.
The Elf ban on homosexuality is merely to ensure that there are doomed romances to keep Eru entertained, since that way is more self-sustaining than just declaring couples star-crossed.
The Arda furthest in the timeline is full of completely and utterly traumatized people. The Maitimo that was reembodied was so broken that he begged to be erased from existence.
There is an instance of an evil Maitimo and an evil Feanor. That makes the peal and associates wonder if there is an evil Bell.
What evil Maitimo puts Findekáno through basically amounts to torture and oath-based brainwashing not to mention the murder of Findekáno's family.
As utterly horrifying Angband was for Maitimo, it was arguably worse for Beka. Who displays as a typical victim of systematic abuse, being completely unfazed by the idea of being tortured, raped and mind altered daily.
All instances of Rebecca have a template attraction of being victims of sexual abuse, having no support system or option for justice. They also become pregnant from rape and are forcibly separated from their beloved child. This is made even more horrifying by the fact that in their settings the Rebeccas are just another statistic. Luckier than most in fact, due to their template attraction with Maglors.
The planet of Amenta is a fucking nightmare to be a member of the Red Caste.
Calado Peka is violently assaulted by jealous purples and grays when her relationship with Macalaure becomes public. Maitimo does nothing to punish the perpetrators afterward so Macalaure is driven to kill one of them.
The huge dementor attacking Jenny and the girls.
Vampire Charles Reed, Tamara's lover and father of her child, quite deliberately by starting with gaslighting her and eventually kidnapping their son and turning him into a vampire.
In Room of Requirement, it's revealed that the Demon Cam that holed Valinor worst memory was the exact moment after the fact where he was despairing over the notion that someone might come have come up with a better solution seconds too late.