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Nightmare Fuel: Arrow
Not even counting the shipwreck, Oliver's castaway ordeal was nightmarish before he ever reached the island. Namely on the life raft, Oliver watching his father murder another man and then kill himself. Oliver spends countless days floating aimlessly a few feet from his father's sun-bloated, near-headless corpse as the water slowly runs out. He finally reaches the safety of the island's dry land, only to watch seagulls feasting on his father's rotting body. Even if Oliver had been rescued the day after he reached the island, he already experienced enough horror to justify spending the rest of his life gibbering in a padded cell.
The effects of Vertigo when put in the jugular. It puts the brain's pain receptors on overdrive, causing a crippling amount of psychosomatic pain that can last for days before the person's heart gives out.
The Undertaking: A bunch of rich, powerful people decide to "fix" their city by making the poor side of town the worst it can be, probably to justify whatever renovation efforts they care to make. In the process they do things like corrupt the legal system, stop gentrification projects and shut down free clinics and other social support organizations. If you're one of the poor people harmed by this project, oh well...
Turns out they were actually trying to save the city, but the Undertaking is now bringing down the Glades with bombs to make it look like a freak earthquake destroyed them!
Malcolm's Suddenly Shouting moment right before the Undertaking takes place, which somehow crosses this with Tear Jerker. It's both completely tragic and shows how completely crazy he's become since his wife's death.
Deadshot. Even if you survive with just a graze, you'll be dead from the curare poison he laces his bullets. And good luck using bulletproof glass. He'll just shatter it and fire another round immediately after.
The Island as a whole. With WW2 mines laying about, armed mercs who'll grab and attack anyone they see and torture for info they may not even have, the fact that both a crashed plane and two boats have ended up there like the Bemuda Triangle, and the questionable implication of what, exactly, the hell are they even doing there. The realization that it was able to transform a shallow party boy into a hardened killer in mere years will make you think twice about any trip over seas.
The Doll Maker. The first victim we see of him is treated in a more gruesome, CSI kind of way and when Quentin sees the body, he's visibly disturbed. The killer himself is creepy as hell, and takes disturbing pleasure in drowning his victims and turning them into his dolls.
"Broken Dolls" also gives us Slade getting hit by one of the pirates' mortars and set on fire. It is later shown that the damage caused an extremely nasty burn on the entire right side of his head.
In "Crucible" Sara Lance says, "No woman should ever suffer at the hands of men." Some fans have pointed out that this may be an oblique reference to being raped at some point during the six years she was missing. At any rate having spent the time on Amazo, Lian Yu, or in the League of Assassins she's likely to have been through some terrible things - equal to or greater than Oliver.
League of Assassins has her, when she's first found by the Amazo, being dragged out in front of several men in nothing but a shirt and her underwear, with the obvious implication they were planning to gang-rape her. Dr Ivo saves her from it, but she's left terrified and shaken, so much so she flinches when he locks the door (not to stop her leaving, mind, to stop the others coming in).
Speaking of such, the League of Assassins themselves; highly trained assassins capable of catching arrows without flinching, almost immune to pain. Their ranks are large and they can come in, take out their target, then leave without ever being seen, with only rumours of their existence in the far east, despite them operating throughout the world. They transform people with no combat skill prior into ruthlessly efficient killers, in less than two years time. And given that the show is now embracing super powers, its possible Ra's Al Ghul may in fact keep his immortality here, making him all the more scary.
"The Scientist" confirms that yes, Professor Zoom does exist in this universe, and he's already jumping around Barry Allen's timeline.
From "Three Ghosts" Oliver's hallucination of Slade attacking him, especially when we learn the hallucinations are purely psychological. He gets beat up, tables are tossed, and thrown into a glass case, and all the time we know that he's doing it to himself.
Plus, this scene is highly reminiscent of Robin's hallucination of Slade (also thought to be dead) in Teen Titans, and it was mighty effective then as well.
The Count. He was creepy enough to begin with, but once the Vertigo takes affect, he's left completely insane and terrifying on a whole other level. Even worse, he's played by Lincoln!
What happened to Sebastian Blood's parents: he killed his father and framed his mother for it, putting her in a nuthouse. When Laurel sees his mother, she seems to be in a Madness Mantra. When Laurel mentions Sebastian's name, it snaps her out. You know something's messed up when your own mother calls you the Devil.
In "Blind Spot", the mind games that Blood plays with the already fragile Laurel are terrifying, to the point that she can no longer trust herself.
From "Tremors", the effect that Mirakuru has on its victims. Roy goes to see Thea and begs her to leave the city. She asks for a reason, and within ten seconds he gets frustrated and starts unconsciously crushing her arm. He realizes what he's doing and stops before she's hurt, but it's still very disturbing both to Roy and the audience. In the flashbacks, Slade nearly blows up their only way off the island in his desperate mission to avenge Shado, and almost kills Ollie.
William Tockman, the Clock King, in "Time of Death". A meticulous planner and creepy master thief who, regardless of his ostensibly noble motivation, will stop at nothing to get what he's after, and even hacks the Foundry's computers and makes them blow up while Felicity and Diggle are still inside. Talk about Paranoia Fuel.
The ending of "Birds of Prey" is pure Adult Fear. Thea goes out alone at night and Slade drives up to offer a ride. And for more Mirakuru-related goodness, we have Roy crushing the shoulder of a guy who bumped him and refused to apologize. Listen closely enough and you can hear crunching.
"Deathstroke": Back on the island, Slade hallucinating Shado when he has Oliver and Sara at gunpoint. Given the role she played as a Morality Chain in past, you might expect her to be the conscience. Nope, she's advising Slade to keep them trapped on the island and go after their families. What's worse? He's still seeing her in the present day.
Same episode: How did Slade get off the island? He swam until he reached land. Not so bad? He got pulled under by the current and crushed against the rocks, which apparently cut his flesh to the bone. He survived and healed thanks to the Mirakuru, but Jesus that sounds painful.
Adding to this, Slade had one eye exposed to saltwater, thus making what happened to him more horrendous.
"The Man Under The Hood": Isabel gets Mirakuru. Slade was a heroic, well-meaning person before he got injected, and it made him a dangerous psychopath, but Isabel was pretty much a sociopath before her dosage. Who knows what this is going to do to her.
Slade's surprise attack on Team Arrow, where he's just waiting for them in the lair and completely wipes the floor with them, makes a pretty effective Jump Scare.
Roy, completely hopped up on Mirakuru and reduced to a near-mindless state, going on a violent rampage and curbstomping everyone who's unlucky enough to come across him, often involving him breaking their limbs and the violent, stomach-churning snaps that go with them. One cop isn't even that lucky, as Roy proceeds to violently shove one of Oliver's arrows right through his gut.
His hallucination of Thea, who begs him to kill her.
This one crosses over with Tear Jerker: Slade's Sadistic Choice and murder of Moira right in front of Ollie, while he's tied up and helpless to do anything about it and his sister is sobbing and crying the whole time.
Which extends into his murder of Moira. If the narrative was from his perspective, then we would see his remorse and hesitation challenged by the hallucination of Shado, who insists Moira be killed. Slade acknowledges Moira's courage, but Not-Shado insists she be killed because sparing her wouldn't teach Oliver anything. Notice how his acknowledging her courage is spoken with a wavering voice, with a look of regret. Now compare that to his tone and expression right before he drives his katana into her heart. He succumbs to Not-Shado, to his darkness, and forces himself to kill Moira for the sake of his mission. But once she's dead, once he see what he's done, his energy is spent, which shows when he frees Thea. He's completely devoid of any emotion, and just leaves.
On Oliver's end, we see him completely break. The promo for the next episode shows he's disappeared on everyone again, just like at the beginning of the season when he went back to the island to grieve for Tommy's death. Unlike Robert's death or Shado's death, where Oliver was forced into a situation he had no control over and therefore had to trudge along to keep surviving, this is a situation he saw coming. It was something he was fully aware of and should have been able to prevent. But once Moira hit the ground and he looked into her dead eyes, he lost it. He was back on the island again. But this time, he'd lost the family he had left behind. To him, there's no hope left. To him, there's no reason for anything.
"City of Blood": Ivo's last "gift" to the world was to purposely give the friendly Peter cancer in order to see if Mirakuru could cure it. With no treatment, the cancer has grown into an easily visible and unsettling-looking infection that's slowly killing him. This makes his own infection retroactive Laser-Guided Karma.
"Streets of Fire": Let's start with the basic horror that a bunch of borderline unstoppable thugs are tearing through the city with the explicit orders to tear as much of it down as possible, and kill as many people as they can.
Especially scary is the fact that, until now, each person who received Mirakuru at least had some kind of goal or order to their actions. Slade is an assassin with a specific plan who seems to have trained himself to control his explosive temper to the point he doesn't just go straight to killing Oliver; Cyrus Gold was operating under specific orders on what to do and carried them out; and Roy is instead a hero in training. These soldiers, however, were just random criminals; they could be drug addicts, petty thieves, or money launderers, but they could also be killers, rapists, psychopaths and sadists, any such brand of evil, and now there's nothing to stop them doing what they can, and while they have their orders, they're unspecific enough to let these people do whatever they want. And there's nothing to stop them attacking anyone they see, or going anywhere they want.