Nightmare Fuel / Fate/stay night [Unlimited Blade Works]

All spoilers are unmarked.

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That mass of flesh used to be a teenager.

• We never get to see it in action but apparently there are barriers that could turn humans inside out so that servants could absorb their mana.
• Rider's death. Although we don't see it in person, we see the aftermath. Apparently Souichirou had Caster enhance his strength enough that he was able to break Rider's neck and twist her head around 720 degrees.
• Illyasviel's flashback: her "mother" appears to her as an amorphous gelatinous and bloody thing, and begins to torture her mentally. Then you notice that blood is spilling everywhere in the room, and it only gets creepier.
• The death of Illyasviel during the second season: after finally disposing of Berserker, Gilgamesh nonchalantly slashes her eyes, making her blind, then stabs her. Shiro is powerless and can only watch, mad with grief and horror, all the while being restrained by Tohsaka, as the poor girl is being tortured, unable to do a single thing to save her. She dies next to her servant, in a pool of her own blood, after having lost everything and everyone.
• Berserker himself is a walking Nightmare Fuel. No matter how many times he is gruesomely impaled by Gilgamesh, he comes back from the dead to fight again, until his Noble Phantasm can't keep up. And then he inexplicably revives himself a 13th time. Even Gilgamesh didn't see that coming.
• Shirou is a case of Nightmare Fuel—not because he is inherently terrifying, but because of what his mind is like due to how twisted he is. He gets stabbed and doesn't even look at the wound, and he offers his entire arm up to save Fujimura without a second thought. Rin gets more horrified by how he thinks all the time. When he explains why he would leap in to save Illya when there was a 100% chance of him failing and dying along with her, the expression on his face is horrifying. It's subtle, but he simply does not look sane when he tells Tohsaka that he just did it to save her. He really doesn't give his own life a second thought—or even a first.
  • Those who have read the visual novel know that it is even worse than this, because the anime doesn't show the thoughts in his head. He is messed up.
• Episode 18. Archer allowing Rin to be taken into Gilgamesh and Shinji's custody even after the later makes clear his intent to rape her as soon as he can. Made even worse by the fact that Shinji's "rape-face" looks rather unsettling.
• In Episode 19,
  • We see some of Archer's backstory, from the very moment he became a Counter Guardian, to him killing many people from afar with arrows. All without an expression on his face. If you look closer at the end of Archer's memories we see him zooming out as he stand on a hill...only if you look closely you see it's actually a literal mountain of dead bodies!
  • Similarly, he basically offers Shirou a sword to kill himself. This REALLY shows just how much Archer hates himself, and by extension Shirou. Before he became a Counter Guardian, he would not have even thought of committing suicide.
  • Rin has a rough time in this episode. First Shinji fondles her legs while she's tied up and gloats about all the things he's going to do to her. That screen of her reaction to this humiliation is disturbing. Then a Hope Spot ensues where her rescuer, Lancer, is under orders from the "fraud priest" Kirei, who first tells Lancer to remove her heart and then commands him to kill himself when he refuses. A second Hope Spot ensues when Lancer kills Kirei but collapses before getting to Shinji, thus leaving her back in the mercy of a rapist. (Though thankfully, Lancer gets back up quick enough to make the creep scitter away) After all that, walking through a burning manor is a major improvement.
• Episode 21: Gilgamesh impales Shinji through the chest with his fist, and inserts Ilya's heart into the hole, turning Shinji into the vessel for the Grail. He totally deserves it, but the process of him becoming the Grail's vessel is beyond horrific. He is mutated into a mass of flesh and limbs, just like Tetsuo from AKIRA, while he screams and chokes in agony. Just look at him right now!◊
• Episode 24: Angra Mainyu's partial manifestation, a grotesque mass of flesh shaped like a humanoid torso, with one glowing slit where its face should be and another in the middle of its chest. Saber also comes dangerously close to stepping into its black ooze, which as Heaven's Feel shows would have blackened her into Saber Alter.