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Nightmare Fuel: Star Wars: The Clone Wars
As Senator Padme Amidala once told C-3PO, it's a big universe out there. That means there's also some horrors waiting that could stick into the minds of even the most fearless Jedi, whether it be the horrors of war, creepy locales, species or psychopathic individuals.
In "Rising Malevolence" (and only the second episode of the series to boot), clones getting their escape pods cracked by the nonchalant droids... humming a jaunty tune. Somewhat creepy coming from the usually goofy B1s/OOMs.
The episode "Lair of Grevious" had a much darker tone compared to others.
The very nasty fight with Grevious that has the Jedi chopping his legs off, while he crawls on the ceiling.
A clone trooper getting incinerated in a lava pit, ala Temple of Doom.
Grevious' horrifying "pet" which mauls another clone trooper to death.
Grievous' droid doctor telling him that there may be "some discomfort" and then beginning to cut open his head as he screams in agony.
In "Cargo of Doom", there's a scene where Cad Bane tortures a Jedi using electricity. The entire sequence - Bane's coldness, the Jedi's death, complete with tongue sticking out, and what really sealed it was the emotionless battle droids.
The scene in "Children of the Force" where Obi-Wan, Anakin, and Mace try to gain information from Cad Bane by using a Jedi Mind Trick on him, all at the same time. It's pretty creepy to watch the cold, calculating bounty hunter thrash in obvious agony as they assault his mind, with the Jedi knowing full well it could destroy him both mentally and physically.
The spider assassin-droid in "Voyage of Temptation". Imagine a giant metal spider with dozens of glowing red eyes, and it's trying to kill you. Now imagine that when you kill it, it starts spewing out dozens of little metal spiders, also trying to kill you.
There's also that one scene of it moving one unfortunate Clone Trooper's corpse like a macabre marionette when Anakin comes across it before throwing the corpse at him.
The Geonosis arc was heavily-action packed for the most part, but the second half episodes ("Legacy of Terror" and "Brain Invaders") took a dive into horror territory.
If you didn't think undead Geonosians were creepy enough, the source of their reanimation is even worse: the Brain Worms, which can not only control dead Geonosians, but living members of other species too.
One of the Geonosians pulls out a mind-control worm and shows it to Luminara. Anakin wishes to free her, but Obi-Wan tells him to wait it out. The Geonosian then puts the worm on Luminara's face and Obi-Wan is still waiting to see how the worm will enter her head, although he really wasn't and was just stalling Queen Karina so Commander Cody's men can get ready to blind her and the undead for their rescue plan.
The scene where Ahsoka and Barriss watches a bunch of infected clones force a brain worm up another clone's nose. It gave a whole new meaning to Mind Rape.
Ahsoka's lightsaber duel with the worm-possessed Barriss, who at one point screams at Ahsoka with the worm coming out of her mouth like a long, snake-like tongue, which also provides the page image.
Anakin interrogating Poggle, the only person who knows the Worm's weakness. Anakin punches the prisoner into a wall after he refuses to comply, then followed by him full on FORCE CHOKING the now victim. It's horrible enough on it's own and with the protocol droid translating everything Poggle says in a Creepy Monotone, but the background music sends it further into Nightmare fuel. The music? The Imperial March.
And guess what? This is only the first of several "Vader moments" for Anakin in this series.
Savage Opress' transformation in "Monster", where his body is mutated into a larger, more powerful form, with his bones audibly cracking as they extend. The change to his personality is no less horrifying: he is turned from a decent, honourable man into a violent, brutish monster, willing to murder his own brother in cold blood with only a moment's hesitation. The murder of his own brother doubles as a Tear Jerker.
Savage's death scene in "The Lawless" lends itself to some disturbing Fridge Horror: as he dies, Savage's body changes back to it's original appearance, and his personality seems to revert back to his pre-transformation self, which raises the question of whether any of Savage's evil acts were his own choice, or if he spent the final months of his life as a slave of the Nightsisters' brainwashing.
The Son from the Mortis arc, who's basically the Dark Side incarnate. He also sounds like Emperor Palpatine at times.
From the same arc, Anakin's vision of his future was horrifying. He's basically watching himself butcher innocent children, fatally throttle his beloved wife, attempt to murder his best friend, and be party to the destruction of a planet and the genocide of billions of innocent people, all while being unable to stop the visions. Forget turning to the dark side, he's lucky he didn't go insane from the experience.
And the worst part? There's nothing in those visions that he cannot prevent.
Ahsoka after the Son brainwashes her into becoming his dark side servant in "Altar of Mortis". Especially the part where we see her eye open, where we see it turn yellow (the colour for dark side eyes).
The torture scene in "The Citadel" deserves mention, especially those six words said by the torture droid as it reached for Even Piell's eye; "Say goodbye to your good eye." Brr...
Going even further, being hunted for sport is a terrifying concept, but these Trandoshans also take trophies from sentient prey, and it's not limited to Wookiees. In addition to animal trophies, that hunting lodge mentioned above includes the heads of an Ithorian, a Gungan and a Gran among others stuffed and mounted on pedestals.
And then we have the not very kid-friendly implication that if Garnac (leader of the Trandoshan hunters) was the one who came out on top in the S3 finale, he would have gone through with his promise to nail Ahsoka's hide to the wall for killing his son. *shudders* And you know how Kalifa mentioned that there were other Padawans/younglings on Wasskhah before Ahsoka came? They were most likely taxidermied after being killed too.
Exploding Gungans, Mon Cala and Quarren, courtesy of Riff Tamson's knife bombs. It gets even more gruesome when Lee-Char blows Tamson to bits with one of the shark-man's own bombs. We not only see blood onscreen, but also some of Tamson's remains, including his head, sink to the bottom of the sea.
Riff Tamson himself is pretty terrifying on his own. The aquatic life forms are all based off of squids, octopus and some kind of amphibian. Tamson? He's a freaking Shark. It's shown that he's fast enough to catch enemies on motorized water skis with just his own swimming prowess, and on top of his exploding knives, it's shown that his own teeth can crack glass meant to withstand the pressures of deep-sea diving. He's also a ruthless and cunning warlord. If it wasn't for his preference for close combat, Prince Lee-Char might have never stood a chance against him.
Even worse is he bought along more of his race to help him in the Prince's public execution, and their method of execution is to eat him alive while chained.
Being from the clones' perspective, the Umbara arc is a study in War Is Hell, especially once General Pong Krell takes command of the 501st Legion.
The planet itself isn't safe either, especially with the existence of one particular creature encountered early in the campaign: the Vixus. Because it's Always Night on Umbara (thus lowering visibility), you might not notice that the vine you just tripped on was the tentacle of a large passive predator, and there's more than one of these things on the planet. Oh, and the inside of its mouth glows. Even worse, it's similar to a more horrific creature in the Star Wars universe: the Sarlacc, aside from the possibility that it keeps you alive while digesting you. Thankfully, the Vixus is much easier to kill.
The Reveal in "Carnage of Krell" that Rex and his men have been killing other clones because of Krell's manipulations.
General Krell's rampage in "Carnage of Krell", as well as his obvious sadistic glee in taunting and butchering the clones.
A more subtle one, but in the first episode of the "Deception" arc, after Obi-Wan fakes his own death to go undercover as Rako Hardeen and insists that keeping Anakin in the dark was vital because "everyone knows how close we are" and Anakin's reaction would sell it. So, Anakin's best friend and Heterosexual Life Partner, who knows Anakin has major issues with rage, grief, and loss, purposefully manipulated his emotions to hurt him to further the mission. Ouch. No wonder Anakin has such major trust issues by the time Revenge of the Sith rolls around. In fairness, Obi-Wan's horror at seeing how much his "death" has unhinged Anakin in "Friends and Enemies" seems to caused him regret.
Actually he might well not know. It is highly likely that Anakin never confided any of his... family issues to anyone but Padme, and certainly he didn't do so on screen. The council's plan was still hardly farsighted or morally sound.
From the same arc, Obi-Wan's transformation into Rako Hardeen, which looked very agonizing.
Almost everything from "Massacre", including Mother Talzin's Cold-Blooded Torture of Dooku via voodoo doll and the Nightsister zombies.
Darth Maul. He's become a psychotic, deranged lunatic, skittering around on a spider-like contraption, ranting dementedly. If possible, his single moment of sanity at the end of his first appearance is even worse; an unblinking, yellow-eyed Death Glare, showing that beneath his insanity, Maul is still a vicious predator, obsessed with revenge.
Darth Maul: Revenge... I must have revenge...
He's laughing and crying at random intervals between broken ramblings interspersed with coherent phrases about revenge, and there's the implication that he is fully aware of his broken mental state.
Darth Maul: The chains... The chains are the easy part. It's what goes on in here that's hard [taps head, begins to sob].
He's no less horrifying after being healed. Now, instead of being a crazy, ranting lunatic, he's a vicious, calculating psychopath who's willing to butcher dozens, maybe hundreds of innocent people just to get Obi-Wan's attention. Adding on his torture of Obi-Wan, the "beyond excruciating" vengeance he had planned, and his sheer implacability makes Maul into, perhaps, the most terrifying villain to appear in the series to date.
The forging of Maul's new legs: the wires from destroyed battle droids fuse themselves to his spine, then the metal is superheated, with the legs themselves being forged from the slag. And through it all, despite being in a magical sleep, Maul never stops screaming.
While it's not shown on screen, it's implied that Maul killed a bunch of kids in "Revenge".
The Raydonian village after Darth Maul's rampage in "Revenge". Having Maul himself there, standing in front of a burning village, makes him look like something that crawled from the depths of hell and only makes the scene even more nightmarish.
D-Squad being overrun by Buzz Droids.
Though it's a quick scene, Grievous' treatment of one very unfortunate clone in "Bound for Rescue". Grabbing him in one foot, before using him to bludgeon another clone hard enough to kill that one instantly, pinning him down while he desperately struggles to get free, then casually crushing him to death as he desperately reaches a hand out to Obi Wan.
The idea of someone like Darth Maul ruling an entire planet.
Darth Sidious in action. The way he moves/fights is inhuman and demonic, his power is enough to make even someone like Maul beg at his feet in terror, everything about Sidious being Satan is true... and he does it all with a smile of sadistic glee with a few Evil Laughs thrown in.
This cannot be said enough. He instantly slams Savage and Maul into windows way off the ground and pins them there. The only reason they got to fight at all instead of getting crushed to death/Force-choked is because he let them.
And worst of all: this man is the same "Good Chancellor Palpatine" we have seen for five seasons. This man is the kindly and parental authority figure for Anakin and Padme. This man rules the Republic and the Confederacy, and the powerful warriors Dooku and Grievous are utterly subservient to him. The Jedi have no idea. That power you see him use? He's masking it-we literally see him every episode.
The episode "Sabotage" introduced exploding nanodroids that were feed by Letta to her husband, turning him into a living bomb. She used him to commit a sabotage against the Jedi Temple. All that was left of the poor guy was his hand.
Doubles as Paranoia Fuel, as you could end up being made a host without even realizing it. And considering who Letta was willing to use.
Letta's claim that a Jedi came up with the plan to bomb the Temple. It's possibly untrue, but the implications are still bone-chilling.
As it turns out, it's true, and it's someone one would've least expected: no less than Barriss Offee, who's grown disillusioned with the Jedi.
The final few minutes of The Jedi Who Knew Too Much. Watching Anakin and Ahsoka confront each other is chilling. And it's only accented by Anakin desperately trying to pacify his understandably pissed off, hurt and confused Padawan, with Ahsoka growing near hysterical to the point that she's outright yelling at him. The episode then ends with Ahsoka jumping onto a freight headed for Coruscant's Underworld. Holy crap.
To make things more disturbing, it's implied that - despite being in on the Sith conspiracy to an extent - the Kaminoans are unaware of the true purpose of the "inhibitor chips" and that Dooku is lying to them too (Dooku said they were inhibitor chips in his conversations with Lama Su and Nala Se; they may not be aware that Tyranus and Dooku are the same person or that Tyranus is a Sith Lord).
Getting a better look at the true nature of the Kaminoans, you get the feeling life for the clones would have been a lot worse without Jedi such as Shaak Ti around. If a clone is seen as defective, they will at best be resigned to maintenance duty like 99 (who was only born malformed), or at worst, be euthanized for having any mental defects. Barring Pong Krell, the Jedi at least have the decency to treat them like human beings, whereas the Kaminoans only see them as slaves and property.
We get more buzz droid-related horror in "The Unknown" during the attack on the medical shuttle. Let's just say you don't want to be meeting those little gremlin-bots out in space. If you're the pilot and they drill into the cockpit, you're already dead.
Let's talk about Anakin Skywalker. You know? The guy with a list of Berserk Buttons and eventually becomes Darth Vader? He's been shown to be pretty scary in the past if you've managed to push one of those buttons (being a slaver, endangering or killing those he cares about, etc) , but his brutal beatdown of Rush Clovis for advancing on his wife in "The Rise of Clovis" is probably his darkest action in the history of the series. If Padme hadn't told him to stop and that he went too far, he probably would have fallen to the Dark Side a lot sooner.
What makes it somewhat more disturbing is that, unlike in the movies, he is shown a lot more as a good person. The series basically shows how you can turn a hero into a villain without mind control, just a slow process that becomes slowly evident as the seasons go on. Many of Anakin's dark moments have some form of reasoning behind them (Force strangle Poggle the Lesser...to get the information to stop a zombie worm outbreak, his freakout over Rush Clovis being partially jealously, partially the fact that the last time he was around his wife, she was poisoned, etc.) If Sidious wanted to see someone else as his apprentice (Mace, Obi-Wan, etc), you'd probably see the same process be applied to them.
In "Destiny", the idea that Yoda of all people has a Dark Side.
Then there was Yoda's trial for temptation in that same episode: he saw the temple in ruins with various Jedi slain, including a lingering shot of Petro. Then he comes across a dying Ahsoka Tano, who asks him if she'll still be able to become one with the Force even after being expelled from the Order. If anything, this shows us he still regrets his part in the Season 5 finale.
Moraband/Korriban is pretty scary, though that's to be expected, seeing as it's the Sith homeworld. We have talking Sith snakes forming a larger snake and some Sith ghosts spouting out their fatalistic philosophy of no life after death (whether it's The Nothing After Death or Cessation of Existence isn't entirely clear), including that of Darth Bane. Sure the Priestesses said they were a part of Yoda's trial, but whether they're fabrications by the Priestesses or a normal part of Moraband that they knew they could use as part of the trial (as is the case with old Expanded Universe interpretations of the planet) remains a mystery. It says something about Yoda when he isn't scared of this stuff and can talk down (or up, rather) to Darth Bane's ghost.