A character has been a bit ... off for a while now. Maybe they just haven't been themselves, maybe they've gone as far as painting a Room Full of Crazy while reciting a Madness Mantra, or maybe they've gone through a Madness Makeover, but it's apparent from their actions that their rational mind is losing its grip and they are sliding inch by inch toward insanity.
The end result of this varies, depending on the tone of the series and who the character is. If the series is dark and edgy, they may go on a murderous rampage. On the other hand, if it's light and soft, it may just be Played for Laughs, becoming a source for their hilarious dialogue and wacky plans. A villain is likely to have a Villainous Breakdown resulting in Karmic Death, while a hero will just have a Heroic BSoD and then get better.
Occasionally, a character will be seen holding back their insanity and generally keeping it in check, until that final straw breaks the camel's back and they finally snap, having a massive Freak Out.
And most rarely, the Sanity Slippage is the effect of being Gaslighted by a villain.
A common way to demonstrate this process is Mundane Horror: when some weird and unsettling details appear in everyday life, this may be the first sign that a character is losing his grasp on reality.
Compare Go Mad from the Revelation, Freak Out, Villainous Breakdown, Room Full of Crazy, Madness Mantra, Laughing Mad, and With Great Power Comes Great Insanity. If this is a backstory it's Pre-Insanity Reveal. Compare Slowly Slipping Into Evil. The opposite of Sanity Strengthening.
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- Bleak Expectations: A few days on a dessert note island drive Pip Bin utterly mad, to the extent that he loses track of time, and considers eating himself, only relenting when he realises this is "probably a bit of an own goal". He quickly recovers when he finds someone to boss around.
- Played for Drama with Harry Biscuit's mother, who after her husband died started believing she was literally a biscuit (named Susan), and dunked herself to death. After that, the Biscuit fortune was taken by their rivals, the Flapjacks, and Harry was sent to St. Bastards.
- Subverted with Pip Bin's own mother, who after the death of her husband starts sitting in linen closets and claiming to be a tablecloth married to a curtain, before moving on to bizarre interior decorating tips (replacing the walls with geese, putting scatter cushions on fire), and then believing she's the host of a cooking show. In Georgian England. Only it turns out she was faking it, and the minute her husband returns she goes back to normal.
- A daily occurrence in Warhammer and Warhammer 40,000; living in an endlessly terrible Crapsack World will do that. Witches/psykers are especially vulnerable to Slippage, thanks to the source of their powers being the home of The Legions of Hell.
- Also appears in Warhammer Fantasy Roleplay, it has a mechanic for going insane and lists quite a nice amount of mental illnesses for which your character can suffer
- Call of Cthulhu is famous for introducing the SAN attribute, which drops a little each time the players encounter a monster or cast a spell, or something.
- Everyone in New World of Darkness, from normal humans to blood-hungry vampires to reality-bending mages, has a Karma Meter. As you slide down the slippery slope, you tend to become a little more unstable with each step, until finally, humans are reduced to raving lunatics, vampires and werewolves go nuts and become meat-hungry animals, mages and changelings fly off the deep end and lose the ability to separate reality from fantasy, and prometheans lose hope of ever becoming humans.
- The Old World of Darkness also has sanity-slipping Karma Meters, though not all splats have them.
- The fan-made World of Darkness game Genius: The Transgression is unique in that players start out insane by nature of their profession, and as they lose Obligation start unconsciously altering reality to fit their delusions.
- Exalted loves this trope:
- The Solars gradually experience this as they become more and more godlike, from both the Great Curse and general detachment from humanity and less powerful divine beings.
- Being close to a Primordial War survivor in the First Age can be dangerous, humiliating, and bad for your physical, mental, and social health.
- Also happens to Infernals. They have access to Yozi Charms. Everything the Yozis are, is made of Charms. Most of the Yozis are insane, and their Charmsets tend to be arranged so that the good stuff with no drawbacks is padlocked by something with benefits that makes you a little crazier. The best example of this is Kimbery, who has an entire Charm tree (built on The Power of Hate) locked behind something that permanently skews the sanity of its user to be either more naive or more vindictive, depending on the chosen variant.
- In Fading Suns, Psychics and Theurgists are prone to "Urge" and "Hubris" respectively.
- Characters in Eclipse Phase are very hard to kill permanently due to cortical stacks and backups, fortunately for GMs there's a mechanic called "stress points" that can cause psychological disorders or permanent catatonia if the PC lets them accumulate.
- Any Killer Game Master worth the title can put this into any game.
- Blanche in A Streetcar Named Desire as well as the film adaptation with Vivien Leigh.
- Sweeney Todd: The Demon Barber of Fleet Street does this with its eponymous character via "Epiphany".
- The title characters in William Shakespeare's Macbeth and Hamlet gets an obsessive variety.
- The title character of The Phantom of the Opera, though your mileage may vary on how sane he was to begin with. But he's definitely completely off the beam by the end, determined to have Christine at all costs, even if he has to blackmail her into marriage by threatening to kill her lover.
- In Shrek: The Musical, Fiona has a moment of this in "I Know It's Today" from waiting to be rescued from a small room in a tower for over twenty years.
- In The Medium by Gian-Carlo Menotti, Phony Psychic Baba suffers increasingly from delusions of someone touching her throat and childlike voices calling out to her as in one of the séances she and Monica put on. She suspects Toby of being the one who touched her, and interrogates him with increasing violence (being a Cute Mute, Toby never answers).
- Burr shows shades of this in the song "The World Was Wide Enough" during Hamilton. It's particularly evident in the line "This man will not make an orphan of my daughter!"
- Sakura in Fate/stay night. It's like everything anyone ever says to her is another stab at her self confidence. And then Shinji tries to rape her one more time, and then he'll tell Shirou about it. Yeahhhh things kind of go downhill from there. Oh, and she was already eating people in her sleep, passing out frequently and also quietly going crazy anyway. She does get better, however.
- Nine Hours, Nine Persons, Nine Doors has Clover, waking up and being forced into playing the Nonary Game. For a second time. She is fine over this. What gives her a dive in the deep end, is Snake's death. Apparent in the "Axe" ending.
- Higurashi: When They Cry has this occur several times, almost once an arc, and often bloodily.
- The sister series, Umineko: When They Cry, reveals in the fifth arc that Natsuhi has slowly been losing it since Kinzo died two years before the story begins.
- Arc 3 of Umineko also has Eva/Eva-Beatrice really losing it, first after finding the gold and then after Hideyoshi is killed. By the end of the arc, she's gone completely apeshit and shoots Battler.
- Grisaia no Kajitsu: In her bad ending Makina goes through this after watching Yuuji die right in front of her. She proceeds to keep his rotting body in a trash bag talking to him, believing he is still alive.