This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Demon's Souls
The entire premise and backstory could count. Fighting off extremely powerful demons that devour people's souls, leaving them as insane husks? The fact that 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 most of the areas are Nightmare Fuel.
The Tower of Latria is terrifying. The backstory is the tower used to be a place of learning, lead by a king and a queen. The queen exiled her husband, who later returned as the Old Monk, and filled the place with demons, likely killing his wife. 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 hugely swollen heart can be seen clinging to the tower; when you arrive at his room, you discover that he has been dominated and drained body and soul by his Yellow Robes, a demonic artefact much too powerful for him, which leave his dry husk to fight you as a Black Phantom.
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 spider-webbed sky as they swoop down towards you. The enormous, beating heart in the central tower doesn't help the ambiance 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. A plague swamp filled to the brim with trash, filth, and aborted 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 Maiden Astraea, conditions actually improve, because someone finally gives a damn about the soulless within.
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.
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 know 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. 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.
A lot of the bosses aren't evil, merely corrupted by demons. Oolan, Alfred, and Metas were noble knights betrayed by the king. The Leechmonger was formed when the leeches of the Valley of Defilement devoured Risaia, while the Colossus was born of Vito, the Maiden Astraea's companions. Astraea herself went mad with grief, and succumbed to demonic power. The Old Hero was denied the afterlife for some unknown crime, until his soul was demonized. In general, with the possible exception of King Allant himself, NONE of the humanoid bosses began as demons. So what does that mean for the demon slayer who defeats them?