This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Originals
Marcel's methods of punishment, burying other vampires alive in stone, were they starve and desicate.
The reason why Davina is in hiding, her coven holds a ritual that require the murder of their own children for power. The children are led like lambs to the slaughter into a graveyard where they are butchered by their families. When Davina panics and pleas for her mother, all her mother does is look away with an icy stare. Thanks to Marcel, she was saved, but the witches are still looking for her. Gets scarier when you realize that the witches think they are in the right.
Made even scarier, in this troper's opinion, when we realize the Harvest actually works, which means the witches have no reason to think they are not in the right.
The curse on Hayley's family is almost the polar opposite of the usual werewolf curse - they are stuck in their wolf form except on days and nights with a full moon.
The fact there is a city where vampires are in charge, where in Mystic Falls vampires are on the fringes of society. In the French Quarter they feed off tourists with impunity, they can hold trial over a witch in public and execute her, leaving her body in the open without fear of reprisal. When the sun goes down, they own the night.
Rebekah's werewolf venom-induced hallucinations while Genevieve has her trapped in an old sanitorium. Visions that plague her include but are not limited to: an old man throwing up blood and glaring menacingly at her, a partially decapitated woman, and twins that might as well be the brothers of those little girls from The Shining that make the most godawful scream. And that's not even the worst of it! The most horrifying being in that hospital for Rebekah is Klaus, after he learns that she'd used Genevieve to summon Mikael to New Orleans to kill him. He stalks her through the hospital, Papa Tunde's knife in hand, roaring her name with fury as she tries to get away...
We finally get to see what happened on the day Mikael came to New Orleans to kill his children. First, he'd physically attacked Elijah and kidnapped Marcel. Then he breaks Klaus in the middle of a crowded opera house by telling him that he will make sure his memory is forgotten long after he's killed the 'abomination' he thinks Klaus is. The crowning moment comes when the show is about to start, and the stage is filled with the impaled and crucified bodies of Marcel and the people Klaus had just made peace with. And then the audience started laughing. Mikael had compelled the human audience to laugh at the horror and think it was just the best thing ever. And after the Originals had fled, he burned down the opera house. With all those people still inside.
And considering what we know about compulsion, who's to say that those people weren't aware of what was going on, but were unable to do anything but laugh while Mikael burned them all?
The hex that the witches placed on Cami's twin brother, and now on her uncle, Paster O'Connell. We get to see the effects of the hex over time. Kieran is aware that his control is gradually slipping away, that one minute he could be fine and the next he could go on a massacre, starting with his own niece. Worst of all, there's no way of reversing the hex once it's been cast.
With the Other Side falling apart, Mikael can affect the land of the living. The first thing he does is torment his sons (and presumably Rebekah, wherever she is) in their dreams. Then he steps it up by almost strangling Hayley to death while she herself is nearly dying from Monique's voodoo attack. And now he's enlisted Davina's help to bring him back to life.
Hayley's screams of horror and fury and helplessness as she's taken by the witches. She's gone into premature labour, and as she's struggling through the pain, she's told that the witches are going to sacrifice her baby to their ancestors.
Just as bad is Klaus' own roar of fury as he searches for Hayley and his child and hears her screaming. When he gets back to full strength, there will be a reckoning.
Esther Mikaelson is still gunning for her children, and it was her decree to kill Hope. It can be argued that her children deserve to die for the horrors they've each wrought since they were turned, but a little newborn baby? Her own granddaughter whose own fate hasn't been set in stone? And now, like Mikael, she's escaped from supernatural purgatory, determined to finish what she started.
It gets worse in the second season: Esther's plan is to undo her children's vampirism by putting their souls in mortal bodies, like she did with Kol and Finn. She wants them to do this voluntarily by - in her own words - systematically destroying everything and everyone that they have ever cared for. And the scary part is she's doing this out of love. Esther loves her children so much that she's going to emotionally, psychologically and then physically rip them apart to 'save' them from the vampirism she put on them. She honest to God believes this.
And she starts with a bang, capturing Elijah and mentally and emotionally tearing him down while he's incapacitated: first with the knowledge that he was the one who killed Tatia, not Esther. And then she traps him inside his mind, in a dream world where Hayley comes to rescue him, and he loses control again.
The fact that Klaus thought it was a good idea to undagger Kol while Marcel was still a child and living under his roof. Elijah's look of abject horror as Klaus merrily shows him the dagger speaks volumes.
Kol using innocent bystanders to re-enact a Shakespeare play, complete with murder and a horrified, hostage teen Marcel as audience . Even worse, he was planning to turn Marcel. As a vampire, all emotions are intensified: what would that have done to a vampire forced to spent eternity as a young teenager, never growing up?
Finn's plan to use untriggered werewolf children in his war against the Originals, by having them murder the human opposition in New Orleans under a blood moon to activate the curse. None of them look older than thirteen.
The introduction of Esther's older sister Dahlia, who used her witchcraft to help Esther conceive...in exchange for the life of Esther's first-born child and the first-born of each subsequent generation that carries her bloodline. If Esther reneges on the deal, Dahlia swears to come back and take all her children.
It gets even worse. Esther is a thousand-year old witch of great power. And yet she's flat-fucking terrified of Dahlia, even more than she is of Mikael, to the extent that she was going to kill her own granddaughter in the season one finale to prevent her from ever returning.
Pretty much everything that's revealed about Dahlia makes her even more frightening. She takes a little girl from her mother and trains her to be a witch that she can siphon power from, only 'caring' for her in the sense that she can't draw power from her if she starves or gets sick. She destroys whole villages for the tiniest slight or perceived threat made against her. After learning that Esther turned her remaining children into vampires, she planned to make Freya have children of her own and 'train' them like she did to her. She forcibly binds Freya into a spell that renders her completely invulnerable even from death, not even letting her have that escape from her. She even kills the first and only man Freya ever loved, and when Freya's unborn child was killed in her suicide attempt, her only emotion was regret that she lost another witch to draw power from. And now she wants to inflict the same monstrous abuse and torment to Hope.
Elijah slaughtering customers at the diner where he meets Rebekah, in a paranoid bid to ensure that no-one knows Hope is alive. Rebekah is so disturbed and horrified by this barbarism from her brother - who would normally compel people first before even thinking of wholesale slaughter - that she snaps his neck to make sure he doesn't lose it near the baby.
Even before this event, Elijah shows signs of losing his control when confronting Kol and Finn. Klaus is the one that has to restrain Elijah, a sure sign that whatever Esther did to him was more serious than originally thought.
Klaus' punishment for Finn? To lock him in a coffin. This, after spending 900 of his 1000+ years as a vampire daggered and locked away. Finn's screams of horror continue to be heard after Klaus and Marcel walk away.
Kol does the same thing to Rebekah but on a more horrible scale. He interferes with Esther's spell so her soul jumps into an unknown woman instead of back in her original body. Said woman is trapped inside the old Fauline mansion, barred by powerful magic from ever leaving.
Finn going off the deep end when he discovers Esther chose to become a vampire. He hasn't shown any signs of slowing down, jumping quickly from wanting to get rid of his siblings to wanting to eradicate vampires and anyone who helps them. And now he knows that Hope is alive.
The revelation that Freya Mikaelson is still alive, fully grown, and every bit as powerful a witch as her mother and aunt. So far, she seems to be on the side of her family (at least, Rebekah's side), but knowing how dysfunctional the Originals are, how long will it be before she comes to the same conclusions about her siblings that Esther did?
There is something unsettling about Hope being able to tap into her witch and hybrid powers before her first birthday. Her creepy, focused gaze at Camille doesn't help matters.
Eva Sinclair, a witch who steals power from child witches by kidnapping them and slowly killing them. And then poor Rebekah ends up stuck in her body, and having to deal with all the people whose children she killed. And if that wasn't enough, either Eva herself is still in there and can take control, or Rebekah's own mind is being poisoned by being in her body.
Finn's fate in 2.15. Namely, having his soul forcibly removed from the body he's possessing and trapped inside Freya's pendant for an indefinite amount of time. Hopefully, he's not conscious of it. Hopefully.
First he drains Marcel of his vervain in order to compel him into doing whatever he wants, including killing Rebecca if she tries to escape (the result of which means she ends up killing herself to go back to her original body, thereby being screwed out of a chance at having children of her own again. This is after he murders every single one of Marcel's vampire brethren, who were mostly innocent and had nothing to do with the war going on at the moment.
Klaus then next subjugates Elijah and forces him to watch a compelled Gia burn herself alive, all the while smirking at Elijah's attempts to break free and his agonizing screams and cries for Klaus to NOT go through with it. Elijah then loses his composure utterly and attempts to attack Klaus, only to have Papa Tunde's blade be driven into him for his trouble. Cami witnesses all this and begs Klaus to stop. She gets bitten and nearly drained of all her blood for it.
Finally, and the most jarring, is Klaus screwing over Hayley, Jackson and the rest of the tribe over by having Dahlia activating the Crescent Moon curse again, this time on Hayley. Being linked to her, the tribe will now share the horrible fate of being stuck as wolves for everyday except the full moon. The transformation for each looks and sounds very painful, not helped with Hayley trying to appeal to any sort of decency left in Klaus to NOT deliver their daughter to the hands of a psychopath. Klaus responds coldly that she already tried doing that to him by running away with HIS daughter and that she deserves not being near baby Hope.
While Cami reveals that the entire episode of Klaus's actions were simply to get Dahlia to let down her guard and finally kill her, the fact that Klaus would go to such lengths to do so is disturbing. Even if they somehow triumph over Dahlia, the cost at doing so was too great; Klaus essentially alienated his surrogate son (Marcel), potentially turned his siblings against him irreparably, murdered innocent bystanders just to hurt those who "wronged" him (Gia) and stripped his daughter from his mother, all because no one listened to''how things he says'' should be done and that the only one in ''charge'' around the place is HIM. And people wonder why Hayley wanted to run away and raise her daughter away from such a paranoid, narcissistic monster?
The aftermath of Klaus' gambit plays out in the finale. Rebekah is given the chance to return to Eva's body and takes it, and Elijah has had enough of Klaus and walks out of his life. He only returns after Hayley pleads with him to raise Hope as well, so that Klaus isn't the only father figure she knows in her life.
Esther and Dahlia's ultimate fate, if only as a statement about the universe of The Originals and The Vampire Diaries. Esther's plan throughout the entirety of season 2 was to put her children in other bodies, stealing the lives of others for her children. Dahlia unapologetically subjected Freya to slavery, and fully intended to do the same to Hope. After they die, what happens to them? They find peace, essentially going to Heaven. After all they've done, they get to find eternal peace together.
Seeing Kol being just as psychopathic when he was newly turned, as he was centuries later. The others took time to develop their complete lack of empathy, but Kol...was that bad right from the start.