The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far...
Abuse Mistake: Dreamlands adventure "Pickman's Student". When the investigators go to the apartment of Mr. and Mrs. Briggs, they find a man tied to a chair with a bathrobe thrown over him. If they rescue him they discover that he's playing bondage games with his wife.
Apocalyptic Log: Shadows of Yog-Sothoth adventure "The Warren", Horror on the Orient Express campaign, Cthulhu Companion adventure "The Mystery of Loch Feinn", and the Fearful Passages supplement adventure "Armored Angels".
Apothecary Alligator: In the campaign The Fungi from Yuggoth, adventure "The Thing in the Well", Dr. Cornwallis has a stuffed alligator hanging by wires from the ceiling of his alchemical laboratory.
Arrested for Heroism: Noted as a potential problem for Investigators in the Cthulhu Companion and the "Field Manual of the Theron Marks Society" (Terror from the Stars).
Attack of the Monster Appendage: Dreamlands adventure "Yellow Sails". When Mironim-Mer activates the Oracle Mirror, the wendigo demon possessing it sends out a tentacle to grab him.
Bad Habits: The Asylum and Other Tales adventure "The Mauretania". A Bolshevik agent pretends to be a priest to get close to a Russian count for an assassination attempt.
Bedlam House: Creepy insane asylums are a recurring setting. It's even possible to visit (or more likely be locked up in) the original Arkham Asylum.
Bedouin Rescue Service: Curse of the Chthonians, adventure "The City Without A Name". After the investigators leave Irem, if they run out of camels and water in the desert they can be rescued by a small band of Bedouins.
Being Human Sucks: Cthulhu Companion, adventure "Paper Chase". A book lover named Douglas Kimball hates dealing with people. He meets some ghouls, goes to live with them and eventually becomes one himself.
Big Creepy-Crawlies: Leng Spiders, Insects from Shaggai, and others. This is no game for the entomophobic.
Blessed with Suck. One old issue of Wizard Magazine stated that Call of Cthulhu is the only game in which the player with the fastest speed lives the longest, and the only game in which no one wants the magic item. It doesn't help the original game was a ticking countdown from sanity to insanity: your character will go insane. It's just a matter of how fast. Of course, your character might not go insane. They have a good chance of dying before they hit that point. Needless to say, ending a Call of Cthulhu game with a living and sane character is unlikely at best.
Blue and Orange Morality: Even the most superficially benevolent races in the game have utterly inscrutable or bizarre motives.
Body Horror: Mostly for non-player characters, but PCs aren't safe either.
Brain in a Jar: A possible fate for characters who get on the wrong side of the Mi-Go.
This does not mean you've pissed them off. Then they just kill you. They stuff your brain in a jar if they like you.
Unfortunately, the Mi-Go are just not all that good at emulating human senses (which, given they are sentient fungus-things, makes sense). One supplement posits that the use of speech software, high quality cameras, microphones, and a lot of lucky rolls would allow a brain in a jar to have a lot closer to human abilities.
Brotherhood of Evil: The Brotherhood of the Beast in The Fungi from Yuggoth (and its remake Day of the Beast).
Butt Monkey: Harvey Walters, the sample character used for gameplay examples (i.e. being maimed/driven mad).
Cannibal Larder: In the supplement Shadows of Yog-Sothoth, adventure "The Worm That Walks". When the PCs explore the Woodie house they discover a kitchen which is the butcher shop of a den of cannibals. It has the gruesome remnants of an earlier meal spread around - hands, feet, and even more grisly bits of human debris.
Celestial Body: The Fungi from Yuggoth, "Day of the Beast". When the Sphinx turns into the Beast (an avatar of Nyarlathotep) its face falls off, revealing a black oval void filled with whirling suns and galaxies.
Cargo Cult: Glozel Est Authentique!. When Phoenician traders stopped coming, the people in the area that would become Glozel created tablets with Phoenician characters on them to try to bring them back.
Cat Girl: Bast, the Egyptian goddess of cats. She shows up to exact vengeance if someone hurts a cat.
Cats Are Magic: A staple of Lovecraftian fiction in his "Dreamlands" works, naturally replicated here and with the inclusion of the goddess Bast as an Elder God.
Cave Mouth: Fragments of Fear adventure "Valley of the Four Shrines". The entrance to the cave that leads to the Valley has been carved in the likeness of Cthulhu's head.
Cigar Fuse Lighting: Terror from the Stars, "Field Manual of the Theron Marks Society". Theron Marks always had a lit cigar in his mouth during adventures in order to light sticks of dynamite.
Concealing Canvas: In Utatti Asfet and The Asylum and Other Tales adventure "The Asylum".
Cool Uncle: In The Fungi from Yuggoth adventure "Mountains of the Moon", an NPC named Victor recognizes one Player Character. He was an old friend of the PC's father, and the PC remembers him as "Uncle Victor", a warm, good-hearted man (even though Mom didn't seem to like him).
Deadly Hug: Masks of Nyarlathotep. M'Weru likes to embrace her victim, cast the Hands of Colubra spell (which changes her hands into the heads of poisonous snakes) and use them to bite her victim and poison them.
Dead Man Writing: Dark Designs adventure "Eyes for the Blind". Elias Cartwright's note starts "By the time you read this I fear that I shall be dead".
Deal with the Devil: The spell Unspeakable Oath. By devoting yourself to the power Hastur, he grants you a wish or boon. However, you begin to lose charisma at a gradual rate. When your charisma score is 0, or when Hastur wills it, you become his slavering thrall
Departure Means Death: Spectral Hunters must stay within 1 mile of the doll that was used in their creation.
Emotion Suppression: The Asylum and Other Tales adventure "The Asylum". Doctor Freygan has developed a drug called Mood Flattener. It temporarily suppresses all emotion in the recipient.
Evil Mask: Fragments of Fear adventure "Valley of the Four Shrines". A metal mask with Cthulhu's face grants the wearer several powers, but each time it's put on the wearer must make a SAN roll or lose sanity.
Explosive Leash: The Sons of Terror's cranial bombs in The Fungi from Yuggoth adventure "By the Bay Part II".
Exposition of Immortality: Various campaigns and scenarios hinge on the uncovering of the Mythos and its dabblers alien natures. Key example of this trope in use occurs in the Cthulhu 1990s campaign Utati Asfet: The Eye Of Wicked Sight. The Big Bad, Labib, is actually an immortal from the time of the Pharoahs. Examination of artifacts and documents in his sanctum during the latters stages of the campaign can lead to this conclusion being made by the players.
Genetic Memory: The Fungi From Yuggoth adventure "Sands of Time". Both the archvillain and the player characters experience an awakening of ancient genetic memories stored in their DNA.
Giant Spider: In the Dreamlands, Leng Spiders can grow to huge size and weigh hundreds of tons.
Glorious Leader: The Fungi from Yuggoth campaign. The Brotherhood of the Beast plans to cause worldwide disaster so its leader can step in, save the world and be made President of the U.S. (and eventually ruler of the world).
Go Mad from the Revelation: Trope Namer (the phrase is taken from the same paragraph quoted above). When the characters realize just what is going on, their Sanity often snaps. As they learn more about how the world really works (the "Cthulhu mythos" stat), their maximum Sanity permanently drops by an equal amount. Thus, someone with a perfect Mythos score, having learned everything there is to know about the universe, would have no Sanity at all.
Well, while a perfect Mythos score is enough to leave you with zero Sanity, it does not mean you know all there is to know (that would still be kinda tempting). Just imagine trying to fill a glass of water (your mindful of sanity) with a truckload of sand. Horrifying, mind-shattering sand...
Gun Accessories: Terror from the Stars, "Field Manual of the Theron Marks Society". The title Investigator group taped flashlights on top of firearms so they could shoot in the dark.
Guns Are Useless: Nothing you can lift is high caliber enough to even scratch Cthulhu. Even heavy artillery won't stop him for long. In comparison, Cthuga is a sentient ball of intense heat. Firing soon-to-be blobs of molten metal at him kinda tickles. However, they do a bang-up job on human cultists and low-level Mythos minions, though, so if you find a group planning to call forth either of the above, break out something high caliber.
"So what happens if you nuke Cthulhu? He reforms ten minutes later, but now he's radioactive!"
Half-Human Hybrid: The Deep Ones and the various half-human offspring of the Outer Gods.
Healing Factor: Chthonians, Cthulhu, Hounds of Tindalos, Shoggoths, Shudde M'ell, Star Spawn of Cthulhu and Tsathoggua can all regenerate injuries.
High Voltage Death: supplement The Asylum and Other Tales, adventure "The Asylum". Doctor Freygan's laboratory has a tub into which victims are placed in order to turn them into proto-shoggoths. If a living person is in the tub when the apparatus is activated, electrical bolts slam into their body and instantly (and horribly) kill them by electrocution.
Hyde Plays Jekyll: The Asylum and Other Tales adventure "The Madman". Adam Smythe has a split personality. His evil side pretends to be his normal personality whenever it's in control.
Immortality Inducer: Lang Fu's Coat of Life in the The Fungi From Yuggoth, adventure "By the Bay Part I" and Bernardo Diaz's ruby ring in the Cthulhu Companion adventure "The Secret of Castronegro".
Instant Death Radius: Yig's poison kills you instantly if you don't dodge and he hits bare skin. Dholes squash you flat if you don't dodge, regardless of defenses, and any surviving investigators get a roll to see if they find enough of you to bury. Cthulhu doesn't give you a chance to dodge (you're really not intended to go into combat with any of the above). For obvious reasons, Azathoth and Yog-Sothoth both take this trope Up to Eleven. The former, if not properly contained, can and will lay waste to entire planets; the latter has a nasty habit of vaporizing everything in a five-mile radius with freaking energy bolts.
Just Before the End: Supplement Fearful Passages, adventure "Slow Boat". The far future setting where the PCs end up, complete with a large orange dying sun and a population of necromancers and zombies.
Killer Game Master: A necessity in a game where the dead PCs are the lucky ones. However, this does not appear to be the written intent of the game. At least the 5th and 6th edition core rulebooks actually encourage the Keeper to come up with alternatives to simply killing off Investigators, such as having monsters choose to target Non-Player Characters instead, having intelligent monsters avoid stupid direct confrontation, and finding alternative bizarre fates to characters dying outright. On the other hand, despite these recommendations, the fragility of the player characters and the game's attractiveness to a Killer Game Master are quite real, as is the tendency for protagonists in the material on which the game is based to meet unpleasant endings.
Kill It with Fire: Terror from the Stars, "Field Manual of the Theron Marks Society". An "Indian Water Pump" filled with gasoline can be used as an improvised flamethrower. Spray the monster with gasoline, then set it on fire.
Kidnapped by an Ally: Pursuit to Kadath. The investigators are kidnapped by a group of armed men and taken to meet U.S. Senator Harold Lindstrom, who wants to hire them to find his son Nils.
Kind Restraints: Masks of Nyarlathotep adventure "The Derbyshire Murders". Eloise Vane becomes a werewolf on nights with a full moon, so her father and brother lock her away in the castle dungeons then.
Lightning Gun: The Mi-go in Terror from the Stars and The Stars Are Right! and a Yithian (Great Race) in the Fragments of Fear adventure "Valley of the Four Shrines".
Lost In Transmission: The Roman legion scribe's account in the Cthulhu By Gaslight adventure "The Yorkshire Horrors"
Louis Cypher: Nyarlathotep is fond of this. In the At Your Door campaign, he appears as realtor Atley P. North and a saxophonist called 'the Royal Pant'.
Lured Into a Trap: Shadows of Yog-Sothoth adventure "The Worm That Walks". Mr. Edwin sends the PCs into several situations where they're guaranteed to be attacked.
Magic Is a Monster Magnet: Cthulhu Companion adventure "The Mystery of Loch Feinn". While in the underground area of Castle MacLaireag, any spell casting doubles the chance of a lloigor detecting and attacking the investigators.
Mega Corp.: New World Incorporated in the supplement ''The Fungi From Yuggoth".
Membership Token: 'The Asylum and Other Tales'' adventure "The Asylum". Freygan's cultists have recognition symbols: small stones with thongs for wearing around the neck.
Mental Picture Projector: Cthulhu Now adventure "Dreams Dark and Deadly". A dream research institute has technology that can read the dreams of sleepers and project them onto TV screens so others can watch them.
Minor Crime Reveals Major Plot: Masks of Nyarlathotep. The murder of author Jackson Elias by cultists leads to a worldwide conspiracy to open a Gate and let the forces of the Cthulhu Mythos conquer the Earth.
Mirror Monster: Dreamlands adventure "Lemon Sails". The Temple of the Oracle on Sarrub has a mirror which has been taken over by a wendigo-demon that attacks anyone who tries to use it.
Morality Chip: Inverted in "The Fungi from Yuggoth'' adventure "By the Bay". A Mad Scientist uses electrical brain implants that can control human beings, to create terrorist criminals and convert a U.S. Treasury agent into The Mole.
Multiarmed And Dangerous: Many monsters and deities have multiple arms/tentacles, such as Azathoth, Cthugha and Shub-Niggurath.
Mysterious Antarctica: Beyond the Mountains of Madness sends the player characters on an expedition to Antarctica.
Naughty Birdwatching: The Asylum and Other Tales adventure "The Auction". An astronomer has a telescope in his apartment which he uses for his secret hobby, voyeurism.
Neck Snap: Supplement Fearful Passages, adventure "Sleigh Ride". A giganteus does it to Professor Chance.
Never Smile at a Crocodile: The game had all sizes of crocodiles. In the "Valley of the Four Shrines" adventure in "Fragments of Fear" they could be encountered as wandering monsters.
No Immortal Inertia: Bernardo Diaz in the Cthulhu Companion adventure "The Secret of Castronegro", Lang Fu in the The Fungi from Yuggoth adventure "By the Bay Part I" and Omar Shakti in Masks of Nyarlathotep chapter 3 "Egypt".
No Sneak Attacks: Encouraged as protocol for the Keeper in the core rulebook. As the Investigators tend to be mere Puny Earthlings, devouring them in their sleep or when they otherwise can't fight back tends to be boring and anticlimactic. Individual pre-written adventure modules may play this straight or avert it, however.
Now Do It Again Backwards: The Resurrection spell and the spell used to raise R'lyeh in the Shadows of Yog-Sothoth adventure "The Rise of R'lyeh".
One-Winged Angel: Destroying any given one form taken by Nyarlathotep results in his manifesting in his horrifyingly monstrous default shape.
Partial Transformation: Proto-shoggoths in The Asylum and Other Tales adventure "The Asylum" and the werewolf Eloise Vane in Masks of Nyarlathotep chapter 2 "London" adventure "The Derbyshire Monster".
Phlebotinum Overdose: Terror Australis adventure "Old Fellow That Bunyip". The investigators must drive a bunyip upriver by calling "Eleanba Wunda", the name of a terrifying spirit which will appear if the name is said too often.
Pit Trap: Worlds of Cthulhu magazine #3, adventure "The Golden Scorpion".
Place of Protection: Fragments of Fear, adventure "Valley of the Four Shrines". A path surrounding a lake prevents zombies from passing over it, protecting a human fishing village from them.
Planetary Parasite: The boxed set Spawn of Azathoth. If a Seed of Azathoth strikes a planet, it can penetrate it and grow into a Spawn of Azathoth the size of a star. All that's left of the planet is shattered ruins. The former fifth planet of the solar system suffered this fate: its remains are the Asteroid Belt.
Playing with Fire: The Great Old One Cthugha and his Fire Vampire servants can perform fire-based attacks.
Pocket Protector: Jack "Brass" Brady's metal plate in the Masks of Nyarlathotep campaign.
Portal Picture: Appeared in the Dreamlands adventure Pickman's Student" and the Masks of Nyarlathotep adventure "A Serpent in Soho".
Pretender Diss: Cthulhu Companion, adventure "The Rescue". The werewolf Rafe Pelton despises and maliciously mistreats two insane men who want to be werewolves like him.
Quicksand Sucks: Can be encountered in the jungle in the Fragments of Fear supplement adventure "Valley of the Four Shrines".
Rapid Aging: Cthulhu Companion adventure "The Secret of Castronegro", The Fungi from Yuggoth adventure "By the Bay Part I" and the Steal Life spell.
Regenerating Mana: Characters have a number of magic points equal to their POW score. Casting Cthulhu Mythos spells uses up the character's magic points. A character regains 1 magic point each (24/POW) hours.
Retcon: As of the the fifth edition, the game discards the entire Good vs Evil aspect Derleth tried to jam into the Cthulhu Mythos, returning to Lovecraft's original vision.
Riddle Me This: Curse of the Chthonians adventure "The City Without A Name". In order to enter, pass through and escape from a special chamber, the investigators must calculate the five numbers of Cthulhu using the occult science of Gematria.
Right-Hand Attack Dog: Masks of Nyarlathotep, Chapter 5 "Shanghai". Lin Tang-yu's trained pet white gorillas TunTun and Ping.
Rock of Limitless Water: The supplement Terror Australis, adventure "City Beneath The Sands". Power Boy can cause a spring to come out of a rock just by sticking his rangga (ceremonial staff) into it.
Romani: The Fungi from Yuggoth adventure "Castle Dark" and the Dark Designs adventure "Eyes for the Blind".
Really Freaking Huge Sandworm: Dholes. As mentioned above, anyone getting in their way is automatically squashed and killed, and the survivors get a roll to try and find enough remains to bury.
Lots of things can cause sanity loss in the game. Increasing Mythos does this, but so do things like killing someone or burning bodies. The game actually has an "Idea" stat, which is a measure of how good your character is at coming up with good ideas. A high idea score is a very bad thing, because when you lose too much sanity in one check, your character goes temporarily insane; you can save against that by failing an Idea roll, with a success meaning your character has understood the full implications of what he is seeing.
Also, increasing your knowledge of the Cthulhu Mythos can not only cost you sanity points during the learning process, but permanently lowers your maximum Sanity.
Scooby-Doo Hoax: In the The Asylum and Other Tales adventure "Westchester House".
Sealed Evil in a Can: The status of a great many (if not most) of the Great Old Ones, including Cthulhu himself.
Secret War: Glozel Est Authentique adventure "Secrets of the Kremlin''. The Nodens Brotherhood has long opposed a cult that worships Shub-Niggurath.
See the Invisible: The Powder of Ibn Gazi and special camera lenses in the Shadows of Yog-Sothoth adventure "Devil's Canyon".
See-Thru Specs: The Fungi From Yuggoth campaign, adventure "The Thing in the Well". A pair of spectacles allows the wearer to see into another dimension. Each time you use them, you could see a monster that could attack and kill you if you don't take them off in time.
Self-Constructed Being: Dreamlands adventure "Pickman's Student". A Great Old One named Ghadamon takes over the body of a human being so he can enter the waking world.
Single-Biome Planet: Curse of the Chthonians adventure "The City Without A Name". If the investigators are very unlucky they can go through a Gate to the home planet of the Chthonians, which is a Volcano Planet.
Dark Designs adventure "The Menace from Sumatra". Dr. Granger's library has The Dynamics of an Asteroid from Sherlock Holmes.
The Malleus Monstrorum sourcebook derives its name from the German title for the game's Creature Companion. Scott David Aniolowski thought it was such a cool name that he used it for the English sourcebook.
Sickbed Slaying: The Fungi From Yuggoth adventures "The Dreamer" and "The Worm that Walks".
Sickening Crunch: Occurs in the Fearful Passages adventure "Sleigh Ride" and The Fungi From Yuggoth adventure "Castle Black".
Smoldering Shoes: Curse of the Chthonians adventure "Dark Carnival". A man is pulled through the iron bars of a gate by ghouls: when his feet get stuck, they're ripped off, leaving them (still inside his shoes) on the ground.
Soul Jar: Shadows of Yog-Sothoth adventure "Devil's Canyon". The spectral hunters cannot move more than 1 mile away from their kachina dolls, which hold their souls. If a specific chant is conducted over a spectral hunter's doll, it is destroyed.
Speak of the Devil: Terror Australis adventure "Old Fella That Bunyip". To drive Bunyip upstream the investigators must say "Eleanba Wunda", the name of a evil spirit which will appear if its name is chanted too often.
Split Personality: The Asylum and Other Tales adventure "The Madman". An investigator is driven insane by exposure to the Cthulhu Mythos and develops an evil alternate personality.
Spooky Seance: Can occur in both The Fungi from Yuggoth and Pursuit to Kadath.
Starfish Aliens: The Elder Things actually have starfish-shaped heads. Then there are the Mi-Go, the Great Race, Cthulhu, etc...
Summon Bigger Fish: Terror From The Stars, "Field Manual of the Theron Marks Society". When facing powerful monsters, summon another deity or monster that hates whatever's attacking you.
Sundial Waypoint: Terror from the Stars adventure "The Temple of the Moon". In the temple there is a light shaft that allows moonlight to enter, and a pool with a map on the bottom. If the Tablet of the Moon is placed atop the light shaft at midnight under the light of the full moon, the moonlight will be refracted into a bright point of light on the map, showing the location of the main temple of Shub-Niggurath.
Sword Cane: One of the new Victorian Age weapons in Cthulhu By Gaslight was the sword cane.
Taking the Bullet: "The Asylum and Other Tales" adventure "The Mauritania". Bodyguards have the skill Block, which allows them to hurl their body in between an attacker and the person they're protecting.
Tap on the Head: The Asylum and Other Tales, adventure "The Asylum". Dr. Freygan's neck pinch. Also, typical of a "Knockout" attack.
Tear Off Your Face: In Terror Australis, the Mimi are creatures from Australian Aboriginal mythology. When angry at a human they may eat all the flesh from his face, leaving the victim alive but horrendously disfigured.
Thank Your Prey: Terror Australis adventure "City Beneath The Sands''. Power Boy could call animals and ask that they allow themselves to be eaten. If they agreed, he praised their beauty and courage.
Total Party Kill: A definite possibility in case of failure. Even when things don't go disastrously wrong, a high character attrition rate is to be expected. Some players believe that the Total Party Kill is what happens if you're lucky. If you're unlucky? A Fate Worse Than Death.
Town with a Dark Secret: The small Vermont town of Bensamin in the Dreamlands adventure "To Sleep, Perchance to Dream".
Trashcan Bonfire: Fearful Passages adventure "The Iron Ghost". There's one in the 1934 Southern Pacific car.
The Tunguska Event: The main rules hint that Azathoth was involved. Spawn of Azathoth says that it was caused by a Seed of Azathoth entering the Earth's atmosphere.
Two Halves Make A Plot: In the supplement Terror from the Stars, adventure "The Temple of the Moon". The Player Characters and two independent villains each receive 1/3 of the Tablet of the Moon, a piece of gold jewelry that allows the user to find the location of a treasure on a map. The players must either defeat one or both villains and take their pieces or team up with them.
Undead Tax Exemption: The Fungi from Yuggoth, adventure "Castle Dark". Baron Hauptmann transfers his mind into a new body every few decades and assumes the victim's identity.
Using You All Along: Curse of the Chthonians adventure "The Curse of Chaugnar Faugn". The investigators are manipulated into helping a mad professor awaken the title Cthulhu Mythos deity from its statue form.
Vampire Vannabe: Cthulhu Companion, adventure "The Rescue". Jocko wants to become a werewolf like Rafe Pelton. Pelton lies to him and cruelly makes him perform useless humiliating acts.
Vampiric Draining: Byakhee, chthonians, Dark Young of Shub-Niggurath, Shub-Niggurath and star vampires can all drain bodily fluids and/or blood. In early editions regular vampires could as well. Tsathoggua can drain points of Characteristics (Strength, Dexterity, etc.).
Weakened by the Light: Fog-Spawn, Hunting Horrors, Vampires, Nyarlathotep in his "Haunter of the Dark" form, Servants of Glaaki, Dreamland shades and others.
The Worm That Walks: The name first appeared in the Shadows of Yog-Sothoth adventure "The Worm That Walks" as the name of a monster that was not made out of worms. An example called "the Crawling One" appeared in the Shadows of Yog-Sothoth adventure "The Watchers of Easter Island".
You Cannot Grasp the True Form: In their vanishingly rare moments of kindness, Outer Gods may manifest in forms not conducive to Sanity-blasting shock. Or, in the case of Nyarlathotep, it's just another way for them to mess with human minds.