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Nightmare Fuel: Elfen Lied
Don't stare at her for too long...
Elfen Lied is a horror series first and foremost, and even though the amount of blood in it can get silly at times, the series is undeniably nightmarish.
Some of the more disturbing scenes, including, of course, the first ten minutes of the anime. The scene where the cruel kids kill Lucy's puppy is either this or Narm depending on your point of view. The scenes where Mariko tears people into pieces, and causes one scientist's intestines to explode out of her.
Bandou's first fight with Lucy leading up to his Eye Scream.
The graveyard scene where Lucy tears off Nana's limbs, down to her fingers, even after she had been neutralized as a threat.
The bit where Mayu is subject to sexual abuse at the hands of her step father while her mother does nothing to help is both this and Tearjerker worthy. When Mayu finally works up the nerve to complain to her mother about the abuse, her mother strikes her down, and then makes it abundantly clear that she views her own rape-victim daughter as a sexual competitor.
At one point a special response unit arrives at an apartment complex and the first thing they come upon is a children's tricycle with a human head in the basket. Inside the apartment they only find a crying three year old girl in pajamas left alive. Who then rips a policeman apart before they open fire with submachine guns.
The scenes where younger Lucy/Kaede hallucinates and sees the dead bodies of the cruel kids that she killed come to life and mock her, as well as when she is talking to her more insane persona (the 'real' Lucy, self entitled the 'Voice of her DNA') inside her mind, leading to her accidentally attempting to strangle Kouta.
The things that Nousou, a scientist from the later parts of the manga, does to create his perfect Mariko clones, including ripping the spines out of Diclonii and creating many clones that are deformed and faceless. These clones are horrifying in and of themselves, as they have no free will of their own and appear to love Nousou but are revealed to have been fully aware of the horrible way he has treated them all along when he tries to redeem himself by releasing one of the clones from mind control and she kills him.
As he said: "We have a serious need for the spinal cords of these girls." Nousou seems kinda cheerful, and of course what else could be behind him other than said spinal cords (with upper skeletons attached!!). Guaranteed to be one of the most mentally-scarring scenes the manga has to offer.
The severed head and chest in the box, which he uses as a diclonius detector. Except she was still alive and had had her vocal cords cut out so that she couldn't scream. Oh, and there was extra pain being inflicted just to make sure she couldn't attack him with her diclonius arms (Even though it had been stated that she can't generate vectors). And he raped her repeatedly before cutting off her lower half just so 'her body wouldn't go completely to waste'.
The Vector Crafts used as weapons against the Diclonius are made from Diclonius brains and alive.
Doubles as Fridge Horror. Diclonii can sense and even communicate with each other telepathically. One can only imagine the kinds of vibes the other Diclonius in the facility are getting from that thing.
Lucy during the festival after finding out Kouta lied to her. Especially in the anime where the subtitles turn red. "I'll slaughter you. Every damn last one of you."
Anna looks like a huge head that lives within the sea with a constant nonchalant glare. She was turned into this when she was a little girl.
The ending to the Manga is downright nightmarish. Especially with Lucy/Nyu/Kaede's body rotting and decaying near the end as she begs Kouta to kill her. During the final scene, what is left of her body is covered by a jacket as it's too gruesome a sight for anyone to watch. And then it comes off.
The Research Facility, which is a textbook definition of a Tailor-Made Prison. The girls in there are held like animals, much like the Real Life concentration camps of Germany, Japan or the US and Soviet Union during World War II, or the merchant ships of the Early Modern Times where black slaves were held. And one of the favourite experimentation method is to fire cannonballs at them in order to test the strenght of their vectors.
To sum a very lowly opinion of this series up: Puppets, creepy girls, psychopathy, hands, and death.
Let's just say EVERYTHING in this series is Nightmare Fuel.
Kouta's backstory as revealed in episode 12 of the anime: Imagine being a young child and watching your father and sister be slaughtered (and only being spared yourself because the murderer considered you a friend) - all because you lied to her about the gender of your cousin. And then you have to stop her from killing your cousin too. No wonder the poor kid developed Trauma-Induced Amnesia.
This.◊ If you're a human, this is what you faced during the 'Diclonius War'. There will be no groping, no Pokemon speak, nothing; as far as she's aware, every encounter with a human is a life or death situation, and it's her life on the line. She's fighting for her right to live, and she'll kill anyone who threatens that. Then consider the fact that there are hundreds more girls just like her, and they're out for your blood. War is hell, indeed...
Fridge Horror nudges that one step beyond when you consider the ultrasound image of an in-utero Diclonius infant seen around the same time in the series. Consider the girls' rapid aging, and that shadowed monster could possibly have once been that as-yet unborn child.