However the ONLY ones who dare stand up against the Celestial Dragons were Monkey D. Luffy and Fisher Tiger.
Slavery. Slaves are forced to do all their master's deeds until they die of exhaustion. If they try to escape, their collars explode. No matter how bad they suffer, they are forced to smile, to show respect to their abusive masters. In fact, slaves don't even have the right to cry! Even though there is no romance or sexual intercourse in this manga, it is very likely that female slaves are sexually abused (which explains Hancock and her sisters' hatred of men).
In Hancock and her sisters' case, they were even fed Devil Fruits to satisfy their masters. Sexual undertones aside, it can still scare the living crap out of you, because it definitely means their masters enjoyed watching people suffer.
Caesar Clown: the hellish child of The Joker and a Hannya mask, expert of Weapons of Mass Destruction, capable of turning his body into a mass of deadly gas and using his own men as guinea pigs to test his biological weapon all while he plays the good guy's role to them. Now that you know this, think again about all those kidnapped children and let the Fridge Horror in.
It gets worse. It's revealed that Caesar was drugging those children by slipping a stimulant drug known as NHC10 on the candies fed to them, to the point where depriving them of it generates not so pleasant◊results◊. Not only does it cause intense pain (The kids were even writhing in agony !), but it also causes the person to go absolutely apeshit, and start attacking everyone at random. Seeing those cute children who begged the Straw Hats to save them suddenly become like drug-craving, bloodthirsty zombies (Complete with Blank White Eyes) is extremely creepy.
Lest we forget, we have a healthy amount of Adult Fear and Paranoia Fuel thrown in around chapter 688. Children being kidnapped, fed the above mentioned candies, their parents declared lost at sea, while they supposedly need to be treated for a disease. Otherwise kind and gentle children being given military-grade drugs. Hmmm, bet you wanna take candy at the doctor's office now. Oh, and the kind gentle Master who gave you all this wonderful candy? He plans to put you in a room filled with gas.
Oh, and if you happen to have some kind of immunity to his poison gas form? He can just remove the oxygen from the air around you so you suffocate. Sweet dreams!
In general, the lengths to which the World Government is ready to go to "uphold peace" are frightening. For example, when they learned that Roger had a child, they didn't know who the mother was. Their solution was to execute any young or expecting mother who was suspected to be connected to Roger in any way. Not to mention they don't even seem to care about upholding peace that much anyway. After all, not only did they cover the escape of many insanely dangerous Level 6 prisoners from Impel Down following Blackbeard's breakout just so they could save face, but they also let the World Nobles (See above) do as they please.
The Buster Call. With the push of a single botton, entire islands are erased. To give you an example of how nightmarish the Buster Call is, Robin, who not only managed to escape her home island's demise at the hands of the Buster Call, but is also one of the most fearless people in the entire story, is completely terrified of the mere possibility of a Buster Call being unleashed.
We also have the Thriller Bark arc, which featured a gigantic ship shaped like a Haunted Castle that was home to Dr. Hogback, a Mad Scientist who made zombies (including zombiefied, literal Mix-and-Match Critters), Perona, an Elegant Gothic Lolita girl who could summon ghosts that came out of the goddamned walls (and floors and paintings), and Big Bad Gecko Moria, a large, vampire-like man who could steal people's shadows, and briefly traps Luffy's shadow in the body of a humongous zombified giant.
The wacky personalities of the various zombies, including Oars (zombified super-giant) act as pretty effective Nightmare Retardant. Because its hard to be terrified of such very silly creatures... Though the method of creating them and the effect of the sun on shadowless people is very Nightmare Fuel-worthy.
Perhaps less then the others, but seeing that... giant thing at the end of the arc. To put it in perspective, Thriller Bark is the size of an island. At the end of the arc, we see this massive creature, easily dwarfing the entire island, merely staring on. It doesn't do anything, but narraration hints it's why ships disappear in the Florian Triangle.
Even after all the Zombies the scares aren't over. Enter Bartholomew Kuma. Bible toting titan of the Shichibukai. With the other Straw Hats broken and beaten, he was going to perform clean up. Zoro manages to plead that he be taken instead of Luffy. Kuma agrees, but not before laying down the line to just the right music.
Kuma: But before you can take his place, I will make you experience Hell.
He wasn't exagerating. He extracts all the pain and fatigue from Luffy, which is manifested as a red bubble as big as Kuma, and puts it into Zoro. To elaborate, when one tiny little baseball sized blob of the pain bubble was pushed into Zoro, he reacted in sheer agony as if a bomb just went off in his chest, coughing up blood.
The anime makes it even worse by having Zoro screaming in agony.
The Blugoris from Impel Down are freaky. Large, vaguely humanoid creatures wielding giant battle-axes... with no face. Just a small lump for a head and what appears to be a painted-on skull.
Impel Down itself. All prisoners are first boiled alive upon being sent there, and part of the punishments involve walking upon a field of razor sharp blades until your feet are nothing more than shreds. The kicker is that this occurs on the first level. The tortures get progressively worse as you continue further deep into what other characters in the manga have explicitly referred to as 'hell on earth'.
And then we get the oh so lovely image of Luffy covered in poisoned goop, with face frozen in an expression of agony and horror, like he's screaming. Sweet dreams.
As if that isn't bad enough, Ivankov's treatment for it is even worse. Imagine everyone's favorite doofy pirate captain screaming nonstop in agony for two straight days as his body breaks down and regenerates. The fact that he got over it in less than a day is definitely a mercy.
Heck, Akainu in general, especially after he recovers from Whitebeard's beatdown and proceeds to chase after Luffy. The man's a damn tank and nothing is going to stand in his way. Not even the toughest pirates of the One Piece world.
Even scarier : from the same arc, we have the anime version of Luffy's breakdown.
Also in the Marineford arc, in the anime, we're treated to a shot of three Pacifista slowly walking towards the camera, with their faces silhouetted, aside from their glowing eyes. In the background we've got what looks like a large fire illuminating the Pacifista. It's pretty unnerving.
Rewind back into Alabasta, and we all remember the lovely scene of Crocodile sucking all the mostiure out of Mr. 3. Seeing a human whither into a mummy, as Mr. 0 puts it, is very uncomfortable. Made all the worse when he does the same thing to Luffy.
Not to mention Crocodile's systematic breaking of Vivi and the berserker frenzy he whips the rebels and Alabastan army into, to the point that even after everyone is knocked over by a gigantic explosion that would've wiped out everybody if not for Pell's Heroic Sacrifice, they just get right back up with crazed expressions and go right back to fighting.
The fight with Kuma is Grade A Nightmare Fuel and Tear Jerker fuel. You have to watch all your friends slowly (as far as you can tell) die, and all you can do is scream, begging them not to fight back and just RUN so you have a chance of meeting them again, all while their killer has no expression as he does the deed and completely curbstomps them by barely raising a hand. Some of them crying out, begging you for help as you stand there completely powerless against your foe. Finally, he comes to little broken and bawling you.
Caribou. Just Caribou. He is a psychotic homicidal lunatic who prays to God to forgive his victims of their sins.
His swamp Logia powers are really grotesque looking, too. He looks like a puddle of melted flesh, although this can be attributed to being black and white and Oda's art style.
Also the way he was kidnapping those mermaids eerily resembled a rapist assaulting his victim.
Hody Jones' latest form can be this, with that huge, gaping murloc-like jaw.
Hody Jones in general. He's basically One Piece's response to Adolf Hitler...or at least, Hitler if he had been stopped before he could have achieved his dreams of conquering his own country...
And now we have a look of her in the Anime, complete with blood-red eyes just like Hody, which somehow manages to make her even more terrifying.
The guys in Chapter 656 who kidnapped the remaining Straw Hats look pretty creepy with those diving suits.
Wait, there's more: brought to an island half on fire, half frozen, in a nursery-like room full of giant children. There's also the question of "what exactly "divided" that samurai like that?", which doesn't get an answer until Law is reintroduced.
Him doing it to the Straw Hats, is actually presented as a comic moment, since it results in a "Freaky Friday" Flip. But then he pulls off a different procedure to Smoker; we see his actual heart. Not so funny.
Speaking of Fridge Horror, Caesar Clown is an "insane bastard" (Smoker's words) whose primary field of expertise as a Mad Scientist is mass-murder weapons. And he holds kids captive inside his main facility. The question "Just what the fuck does he intend to do with them ?!" is guaranteed to stay in your mind until later chapters reveal why he does so: to transform them into giant human weapons, which he does little by little via repeated experiments on them. Worse, he slips a deadly drug called NHC10 into the candies he feeds them to keep them in check. Withdrawal from the stuff causes the kids to experience severe pain and go into an Unstoppable Rage, basically turning them into candy-craving zombies. Eating too much of the stuff like Mocha did to protect her friends leads to excruciating agony, Blood from the Mouth, and near-death (Good thing Chopper was there to stabilize her condition). That's not even the end of it: Caesar gloats about the fact that his young guinea pigs will be dead in five years if this keeps on going, saying he'll have to get replacements. This is one frighteningly repulsive man.
The Blob Monster Slime released by Clown in chapter 668 isn't a pleasant sight.
It's not just its looks that are monstrous, it's also basically invincible. It constantly releases poisonous gas, so you can't get too close without a mask, and cutting or shooting it just makes it spew out more. Plus, if you touch it, you are instantly infected with its poison, and the effects aren't pretty. So, if you can't attack it with weapons, or even attack at all, what's left? How about fire? Nearly all creatures fear fire, so let's try torching this one. But wait, this thing's made up of gas, so it'll just explode if set aflame. And even though it's not good with water, the poison that it spills in still works. And then it shows off its Devil Fruit ability.
And things just got even worse: after eating a bait made by Caesar, it changed into a huge cloud of gas that petrifies its victims fast as shit. Bonus points for how scary the statues look and the guy begging for help as he's being petrified.
AND IT GETS WORSE.... Caesar mentions that apart from the petrification, it's the same gas as before; that is, it cripples, but the victims live long enough to crawl away when they had no legs. Fridge horror ensues since the gas must take hours to actually be deadly, but now paralyses instantly, effectively meaning that the victims merely die of suffocation a minute or so after getting paralysed. Of course, if the ash is porous, which may well be the case considering it's supposed to be an improvement of the same gas weapon, then they simply stay petrified for a long time becoming increasingly poisoned before it eventually kills them. So the only immediate effect is to petrify alive.
Monet's monster form is actually pretty frightening. Seeing the Cute Monster Girl grow a giant set of teeth capable of biting off a Marine's shoulder.Brrr.
Later on, Zoro cuts her in half vertically. She survives, but that was only because Zoro decided not use Haki. Because Monet is too scared of what Zoro could have done to her, she is unable to put her body back together right away. Seeing her two halves moving about erratically, melting into snow and getting back into shape time and time again, is disturbing as hell. The look of absolute rage on her face(s) is not a pretty sight either, and gets even scarier when you compare it to how calm and cute she looked before.
There's also her ability to weaken someone by embracing them. You have to remember that Monet is a snow woman, and falling asleep in the snow can effectively kill them. Monet does something similar, by weakening an opponent with the cold she exudes and causing them to fall asleep. The fuel doesn't end there though. When she used it on Luffy, she described that his heart would freeze and he would die in his sleep. So Monet basically kills someone with Hypothermia. Urrggh...
You see the toys in Dressrosa? They used to be human. Not only that, but everyone, even those closest to them, forgot they existed in the first place. Worse yet, they can't report on it, because if toys even try to step in a human's house or talk to them too closely, they get declared "broken" and thrown into the scrap bin. That is already scary and sad enough, but to top it off, some of these toys have been in this situation for ten years.
It sort of take a lot of the excitement out of the Tournament Arc that's happening in the Corrida Colosseum when it's revealed that the losers are dropped through a Trap Door into a dark pit where they wait to be turned into toys.
Speaking of taking out the excitement... the entire tournament is shown to be for nought for nearly all the participants given that with said transformation everyone will proceed to lose memories of these humans-turned-toys and consequently the tournament they took part in.
There's also an uncomfortable bit of Fridge Horror: What happened to the broken toys in the scrap heap? Did they turn back, and if they did, in what shape? It's made even worse by the fact that no one mentions it, meaning they possibly stayed transformed if they were dead, or that they didn't manage to escape the scrap heap yet.
Jora is a Mad Artist who apparently has a private art collection she made out of the bodies of those she's defeated in battle. In addition to that, her Art-Art powers lets her make them into any surreal or grotesque form she wishes, and she is that sort of artist. She almost assimilated Nami, Chopper, and Momonosuke into the walls of their ship and has so far proven to be invincible in normal combat: As no weapons or Devil Fruit powers have been shown to work once affected by her abilities, and modified combatants lose much of their physical force. The way she has used her Art-Art Fruit is one of the most unsettling power sets in the series thus far.
Doflamingo puppeteering King Riku into killing his own people against his will. His Slasher Smile framed in shadow does not help matters.
Usopp's face and his scream on the last page of Chapter 742 qualifies. It was so bad, it caused Sugar to pass out from the scare! To some, it would probably be a Nightmare Retardant because of how exaggerated it is, and because Usopp is generally known for his ridiculous wild takes, but a such detailed and distorted face actually looks like something Junji Ito could have drawn in a less comedic context...
The Birdcage. Doflamingo traps all of Dressrosa in a literal birdcage made from his threads. Said cage hurts anyone who tries to leave and cuts off all communication with the outer world. Law then says Doflamingo intends to kill all of Dressrosa before the truth about the country leaks. And Doflamingo says that what will follow will be worse than the time he puppeteered King Riku into killing his own people. Doflamingo reveals he can control anyone he wants that's trapped inside the cage, says he could have ruled over Dressrosa with terror from the start if he wanted to and starts a hunting game where you either kill him to get out of Dressrosa, or kill the people he designates. And the Holy Shit Quotient breaks again.
Movie 6. Plant-men suddenly wilting into dead husks, the Straw Hats being absorbed by a giant plant monster while the villain announces their death, the image of Luffy made into a human pincushion trudging zombie-like while begging for his crew to be returned... it's no wonder the movie is controversial among the fanbase.
One of the worst parts of the absorbing sequence was when Zoro is the last one absorbed. Mainly because, while the others appeared to be unconscious, Zoro has this look on his face that makes one think he's completely aware of what's happening. His mouth is open like he's screaming! What about toward the end when the flower on the villain's shoulder turns into this giant, grotesque beanstalk-thing, and you can see the bodies of Luffy's crew jammed in it?
The cutesy Lily avatar on the Baron's shoulder cheerfully exclaiming that it's hungry, and making biting and chewing motions both during and after absorbing the Straw Hats.