Demon's Souls/Nightmare Fuel


 * The Tower of Latria is terrifying.
 * The Prison of Hope is guarded by eerie, octopus-headed people ringing bells. They kill you by first stunning you, then leisurely pacing over to you and sucking out your brains while you get to watch. The inmates are all insane - at best, they gibber annoyingly and get in the way, at worst, they try to kill you if you let them out. One careless step can send you into the abyss. Oh, and there's a ball of mashed-together corpses on the first floor.
 * The Tower City is almost worse - first, you walk in constant fear of falling off the crumbling towers into the blackness below, but if you keep your eyes on where you put your feet, you won't spot the black gargoyles against the black-green spiderwebbed sky as they swoop down towards you. The enormous, beating heart in the central tower doesn't help the ambience at all. Then, you find a cage-lift to the ground, and you'd celebrate except you're now knee-deep in a swamp of blood with enormous tentacles reaching out of it and it's full of giant, poison-spewing centipedes that try to kill you. Giant poison-spewing centipedes with several screaming human faces each, naturally.
 * The Valley of Defilement's first boss, the Leechmonger, has a ranged attack. What is it you ask? A bunch of leeches being thrown at you! Even after the attack, you get to see your character covered in them! Gross.
 * The entire premise and backstory of Demon's Souls could count. Fighting off extremely powerful demons that devour people's souls, leaving them as insane husks? The fact that there is no god, and hence, if you die, your soul is left behind, laying there until either the PC picks it up, or a demon eats it? How about how Boletaria (the setting's kingdom) is being devoured by a colorless fog that continues to slowly and inexorably spread until it covers the world, filling it with demons? Hell, even some of the AREAS are HONF. Case in point...
 * The Valley of Defilement. A plague swamp filled to the brim with trash, filth, and aborted fucking fetuses that swarm you whenever you go into the water. People used to live here. These were the lost and the luckless souls. Take a guess why. As it stands, when the demons arrive there, courtesy of, conditions actually improve, because someone finally gives a damn about the soulless within.
 * Stonefang Tunnel/Mine. Your average mine, right? Wrong. Demons are here, too. They've eaten the souls of the workers inside; however, because none of them no a life beyond mining and smelting, they continue to do their jobs unless either you or one of them decides to pick a fight. Even being in the area for a long period of time seems to have altered them: they're growing scales, for chrissakes. The reason for the plentiful ore here is because the bones of dragons rest here, including the Dragon God, who they worshiped but apparently expected to come back to life, given they created a sword and a pair of ballista to pin the bastard down. The beginning level of this place will make arachnophobes squeamish; the first boss is a literal Demonic Spider.
 * The Tower of Latria was mentioned above, but not it's backstory. Apparently, the tower used to be a place of learning, lead by a king and a queen. The queen exiled her husband, . The new leader, the Old Monk, lies to the prisoners by offering them a false idol to worship. Said idol is a demon. Apparently, the Old Monk has also become an Eldritch Abomination, because his feaking heart can be seen clinging to the tower.
 * In short: Thank you for sleepless nights in abundance, From Software.