Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: RuneScape
Spoilers for the game below. Venture past this point only if you dare!
In February 2013, in order to build hype for what would be the biggest quest of the year, Jagex released several teasers. The first was a miniquest that involves helping an old friend gain his lost memories back while the second involves taking a trip down to the recently unearthed dungeon where the quest will take place. A third update, which they (possibly intentionally) failed to mention, is the person◊ who begins stalking you the moment you step foot down the stairs into the dungeon.
The Mahjarrat Memories miniquest tells you what one of Sliske's hobbies was during the Second Age. He was a playwright, like Shakespeare. People in his plays died a lot, like Shakespeare. To get actors, he grabbed lower class humans off the streets, put wooden masks on them that controlled their bodies, with the actors literally stabbing each other in the finale of the play. Eventually he got bored with this, and moved on to other hobbies.
The Zarosian Empire also had a nobility which consisted of demons and vampires. The Mahjarrat Hazeel noted that while it was illegal to eat humans, these rules were broken frequently. Much like the horror movie The Purge, the victims tended to be lower class, homeless people.
During the saga Vengeance, the player controls a forgotten warrior whose friends and brother are all slaughtered by a quartet of adventurers (long story). The ghost of Taevas, her brother, comes back to haunt her and to encourage her to exact ruthless vengeance.
"Gut her. Rip her in two. Make her suffer...When she least expects it, crush her. Let me hear her. Let her screams comfort me in the darkness."
Another Dungeoneering saga is Nadir, featuring Lucien's half-human daughter. Zemouregal is interested in "the creation of mongrels such as these". Was Moia created in a test tube, or did Lucien mate with a human female?
According to Jagex, Moia's mother was indeed a human, and apparently a powerful spellcaster at that.
When controlling Moia, examining Zemouregal gets the message that Moia often finds him looking at her.
The Sea Slug quest line:
After you stop Kennith's glowing rage against the poor little girl, he says, with a maniacal portrait box to boot, "Mother Mallum's coming for you!".
Where you're talking to the little girl in the same quests, and the zombie-like villagers claw at the house, saying they want to play with her, come outside, etc...pretty friggen' creepy.
Pretty much Kennith himself. Kennith can float in a glowing red rage, control the slug infected people, and is probably in cahoots with Mother Mallum, if it wasn't stated in the quest. And he's a kid. A FREAKING KID. His parents are afraid of him. That's pretty nasty. At least he eventually grows out of it and helps you take down Mother Mallum in a later quest.
"A Soul's Bane":
In Tolna's Rift, in the second dungeon, you have to look into dark caves to find monsters. If you don't know whats going to happen, a giant cloaked banshee bounces up in front of the screen. It looks a lot like "The Scream." It's even Lampshaded when the hero screams "AAAAAAAAAAAAH!" the first time. The hero, though, gets more and more confident, until they conquer the fear, and clears the room.
The diary of the current owner of the lighthouse in Fremennik Region. It mentions noises from beneath the lighthouse, hearing the waterlogged voice of the previous owner in his dreams, and then he goes to investigate the basement, which is the source of the sounds. There's a reason why the journal ends there. Sweet dreams, dear reader.
On the upside, you do end up successfully rescuing the person in question, who is simply trapped.
While fighting Dagannoths (a race of sea dwelling monsters based partially off of Lovecraft's monsters), they will often drop armor and swords made by the Fremennik tribe. This makes sense, since the Fremennik send warriors to Waterbirth Island to fight the "daggermouths". Dagannoths also drop civilian clothing as a common drop. Yeah.
The entire plot of "Blood Runs Deep", the current most difficult quest in the Fremennik lands, can be summed up as Dagannoth Apocalypse. You start by entering the dreams of the lighthouse keeper mentioned above, only to find out that the dagannoths possess intelligence and are planning to build an army, traits they have never shown before. Then they attack you. Inside the man's dream. Then you head back to Rellekka, which is the main Fremennik city, only to find the dagannoths swarming the docks. After killing several of them, you have to flee the city. Then you find out that the King of one of the Fremennik Islands has been kidnapped (which the Dagannoth Mother from the lighthouse keeper's dreams mentioned doing), and you have to rescue him from a chamber filled with eggs and escort him through the most crowded dungeon in the game.
During the Desert Treasure quest, if you have one of the diamonds in your inventory, you could get attacked randomly by a stranger with a poisoned dagger. The worst part is that it can happen anywhere. Even in your own house.
The Underground Passage quest. It's a long twisting tunnel, filled with various creatures, a supposed demigod of evil, and every so often you get an ominous message. In order to defeat the demigod and stop the voices, you need to make a voodoo doll of him, and throw that doll down the Well of Damnation, which sends people straight to Zamorak himself. When you do so, he will be sent down the well.
The very disturbing messages that appear in your chatbox during your adventures in the Underground Pass. Yep, traps that kill you, monsters that want you for dead, and Mind Rape!
I'll swallow your soul.
At the very end of the series, you find out what was talking to you. It was an aspect of a goddess known for being benevolent.
The quest also features Koftik, who is slowly driven mad the farther you progress through the pass.
The Boss Banter before fighting Leeuni, where she will gleefully describe crushing the monkey colonists' heads and drinking their intestines, before attempting to do the same to you.
Tolna was locked up underground for twenty years. Until he morphed into that ... thing.
Tormented Demons. Think about it. Their flesh is constantly regenerating and burning at the same time. There's metal sealed over one arm, one leg, and their face. The metal has to be excruciatingly hot. Made worse by the fact that in a letter to a player, the Demons actually seem to be pretty good guys, but apparently whatever magic causes them to be in such excruciating pain also causes them to attack wandering adventurers.
Even though it's more due to Gameplayand Story Segregation, they are still alive and suffering even after Lucien is dead, while summoning magic dispels when the master dies.
"One Piercing Note." A previously-stable woman goes insane, believing herself to be the incarnation of a saint, and starts murdering people. With claws. She kills a totally innocent applicant to the order for no other reason than that she wants to cover her tracks, and ends up mutilating the corpse beyond recognition so people think the corpse is her. She kills another sister for the crime of having an affair, and murderers an innocent novice in front of you for the crime of dancing. Yes, dancing. The ending? She either gets kicked off the tower by the player, throws herself off the tower, or falls off it after thinking she can fly.
"Spirit of Summer." The ghosts in this quest look torn and tattered, inhabit a Dark World version of Wilderness ruins, and they can barely communicate with you even with your ghost speak amulet, so there's a sense of wrongness through the whole quest. Finding out what the problem is makes it less scary but also introduces a soul eating Eldritch Abomination to compensate.
Three words: Queen Black Dragon. It looks like some sort of Leviathan, is known to be evil, and Jagex noted it that more than ten King Black Dragons can fit into her mouth alone. Made more terrifying how by she used a siren named Remora for unknown reasons, then torched her alive at point blank range. She has obvious connections with the Dragonkin based on their symbols being seen often on the way to her. Yes, this Eldritch Abomination is serving the ones that curb-stomped Lucien in an earlier quest. Keep note that she will be out later this month as an instance-only boss.
There used to be a line for NPCs in West Ardougne that said "Get me out of this hellhole!" to show the player how awful living in a town where a fake plague is tormenting the citizens and causing real effects to them. Worse still, the Mourners have complete rule over the entire town and occasionally condemn citizens to death in a house with a black x marked on it as a sign that nobody can enter unless they're a Mourner. Just to keep up the facade and torment the villagers. Even worse, the villagers have only one way out: the horrifyingly sadistic Underground Pass. Even if they somehow manage to get through that, chances are they'll be killed by either the elves working for the one who "created" the plague or fall victim to its traps, both inside the Pass and within the forest.
The Order of Ascension runs on this. Their motivation is that they're trying to kidnap humans in the hopes to make a fake Guthix as a replacement after his death. But it gets more horrifying. As you collect the pages of the book and read the the rather robotic dialogue, you learn they spent multiple generations experimenting on humans and leaving piles of crystalized mummies in their wake. And then, after some exploring, you find a giant statue of Guthix that radiates some unnatural energy and seems to be lacking a mouth. These beings still believe they are doing their duty to Guthix.
During the "Death of Chivalry" quest:
The "Examine" text for Sir Owen once he has been revived as a zombie says "High-Octane Knightmare Fuel", referencing this trope. Puns aside, the whole quest is this as Owen, one of the Signature Heroes, has essentially been turned into a zombie. So imagine fighting your friend and ally, who is now a mindless creature, along with other black knight zombies. And when Saradomin resurrects him again, he is brought back with a corrupted arm. Yes, he Came Back Wrongtwice in the same quest.
On top of that, try reading Captain Dulcin's diary and finding Lensig's lovingly detailed account of how she killed him by dissolving him down to the bone, before impersonating him to the rest of the Black Knights, sleeping in his own quarters, and hiding his bleached bonesunder the same bed. "Creepy" would be a huge understatement.
In the finale of the Rise of the Red Axe quest series (the dwarf quests), when you invade the Red Axe base, you find that Hreidmar wasn't planning to attack Keldagrim. No, he had given up, and had constructed his own royal palace as a monument to his delusions of being king. Seeing the Chaos Dwarves brainwashed to act as Consortium heads serving Hreidmar in a building which looked exactly like the actual Consortium palace was creepy. And then, during the boss fight, Hreidmar picked up a certain magical weapon...
Remember back when we found out the fate of Kethsi? How their use of the Stone of Jas brought the Dragonkin upon them, causing them to suffer a Class 5 Apocalypse? Turns out Kethsi is close to Armadyl's home planet, and he is aware of what happened. Imagine our astronomers discovering a world filled with intelligent life, being ravaged by unstoppable monsters. And humanity being utterly powerless to help them from so far away.
By the end of the quest "Bringing Home the Bacon", it's all but verbally confirmed that you, the player, just helped Eli Bacon poison a number of "bacon addicts" and feed them to his pigs. This includes crates full of human bones that quietly appear around his basement.
Kerapac's laboratory is filled with the remains of creatures he has experimented on. To put it into perspective, Kerapac is a creature enslaved by the Elder Gods to protect an Amplifier Artifact, and is driven insane by hatred of "false users". He is trying to figure out a way around the curse. During the associated quest, the player finds out about some of his cruel Mad Scientist experiments. The metal dragons which the player is used to fighting, for example, were created by pouring molten steel on their eggs, which was every bit as painful as it sounds. Then there is what Kerapac did to the White Dragons - he tried to merge them with abyssal energy, which had some unpleasant side effects. And to think the resulting Celestial Dragons actually looked beautiful.
As of Fate of the Gods, we have a wide variety of goodies. For starters, there is the Cradle, a massive volcano where an Eldritch Abomination is constantly having nightmares. Whenever she does, hordes of lesser monsters spawn from her dreams. Beneath her, we have the Elder Halls, essentially a nest for Elder Gods. When the "eggs" start to "hatch", they drain the soul of the world the nest is on, turning it from what Gielinor is now to the Death World that Freneskae is now. And Gielinor's Elder Halls are apparently due to hatch soon. Runescape has gone from being a Medievel Fantasy game to a Cosmic Horror Story.