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Nightmare Fuel: Shin Megami Tensei III: Nocturne
Humanity dies in a flash of light as the Conception begins. And you can only watch.
When the Young Boy transforms you into a demon against your will by dropping a wriggling leech-like creature into your eye, which then burrows straight into your brain. And this is all done from a first-person perspective. Watch it here.
Good lord, Gozu's priestess. The first time you enter Gozu-Tennoh's chamber, you're confronted with THIS. note It stops about 30 seconds in.
Sakahagi, who happens to wear a bandolier made of Manikin faces that he killed and skinned himself, and also goes through those twitchy palpitations that most Manikins do.
Yuko possessed by Aradia. While Aradia's words give her the impression of a cryptic but supportive personality, Yuko's body twitches like a Manikin in overdrive and teleports like mad. It's more or less the picture of insanity, especially since Yuko's face is hidden from view by a near-formless "mask".
Isamu after his awakening in the Amala Network. Not only does he have the same "disembodied empty faces" motif that Sakahagi has, Isamu's extra faces are not clothing. They're growing out of his body.
Also when the second phase of your fight against Noah has a giant, creepy version of Isamu's head sticking out of a giant fetus's body.
Chiaki after gaining Gozu-Tennoh's power. Actually a bit less creepy than Isamu in body, but her right arm being some roiling, seemingly boneless-at-times thing would be enough to give anyone nightmares... of course, her crushing a Manikin's face and tossing him away like a rag doll add a lot to it too.
The angels. Fighting three of the top seraphs is bad. Seeing the ground troopers respond with such joy to Chiaki's sadism? Frankly horrific.
Before the Nightmare System fires and after escaping the Mantra HQ, go back up to the arena where you fought Thor. You'll see a pair of Pretas and a Manikin, who explains that whoever loses to the guards are eaten alive by the Pretas.
Kabukicho Prison. One side's sunny and the only notable thing are the many empty rooms. The another side is chock-full of Manikins being tortured and drained of their Life Energy. Brutally. Routinely. Normally.
Learning the identity of the Merchant and the Killer in the Fourth Kalpa, and a lot of very hard truths about the Manikins' origins.
The Diet Building. The place looks ominous enough already... and then the bizarre architecture and the illusions start kicking in...
The Reveal as to the true identity and fate of Hijiri. He's actually Aleph from Shin Megami Tensei II, in a particularly cruel form of "Groundhog Day" Loop. Live juuuust enough to see the world die, suffer in the Vortex World, die, be reborn in another world as the Conception begins. Lather. Rinse. Repeat. Ad infinitum. And the worst part? He has no way of affecting the outcome of any one of them. Ever.