A few of the edgier games have a Sanity Meter to measure the main character's morale or degree of corruption. Some actions decrease it, some (precious few) restore it. Of course, once you've characterized sanity as a resource, it's a short step to spending it like one, giving the protagonist abilities with desirable results which carry a cost in decreased sanity...
This trope is for the use of Sanity as a strategic resource which can be knowingly spent for gains in other areas.
This is the mental version of Power Degeneration. If all your powers are Cast From Sanity, you are most likely Blessed with Suck, but if there's just the one for emergency use, you can consider it a Dangerous Forbidden Technique.
The justification for why the Sanity Meter takes a hit from using spells can differ. Maybe the Mana source is tainted, maybe the experience of using the magic is itself traumatic, maybe it's a side effect due to human brains being incompatible with magic — or maybe madness is simply the price you need to pay.
Subtrope of Power at a Price and The Corruption. Sister trope to With Great Power Comes Great Insanity, and Power Born of Madness. Compare and contrast with This Is Your Brain on Evil.
In works where permanent sanity loss can be reversed by spending XP, this trope overlaps with Cast from Experience Points.
Other non-traditional and dangerous ways to cast spells include Cast from Lifespan and Cast from Hit Points. See also Cast from Money, which is less dangerous, but still non-traditional and (usually) not something you want to make a regular habit of.
Anime & Manga
- In Puella Magi Madoka Magica, magical girls appear to use attacks that are Cast from Hit Points since the use of magic taints their soul gems. The use of magic actually falls into this category. As a soul gem becomes cloudier, its user becomes more and more mentally unstable until she finally falls into despair and becomes a Witch. Unfortunately, the nature of a soul gem means that its owner is essentially a lich and has to use magic to keep her body functional. This coupled with the Monkey's Paw like effects of their wish makes the transformation all but inevitable.
- In the IDW Ghostbusters, four demons called The Collectors can be summoned to snatch people and trap them in a Prison Dimension at the cost of the summoner's soul and sanity.
- In The Neverending Story, Bastian's wishing power turns out to require sacrificing memories of the human world. It eventually causes to him to undergo a FaceHeel Turn, due a combination of gradually forgetting who he is at heart and becoming Drunk with Power.
- In The Reckoners Trilogy, Jonathan Phaedrus is very careful to limit the direct use of his powers, because of this trope. He becomes increasingly arrogant and What Measure Is a Non-Super? the more he uses it, unless he spends an extended time period avoiding use of his power. Most Supers in the setting don't know about this fact, and are by now beyond caring.
- Saidin, the magic used by males in The Wheel of Time, is tainted by The Dark One, causing inevitable insanity in its users. As time progresses, one of the main characters begins to show the effects of this, becoming schizophrenic, moody, and temperamental; halfway through the series, he seems like a completely different person, though he is under a lot of pressure... The Forsaken also have access to what they call the True Power, an extremely addictive, evil flavor of magic that also has serious psychological consequences; most would only consider using it under dire need.
- Animus magic in Wings of Fire causes dragons to lose bits of their soul with each spell they use, until they become Ax-Crazy like Albatross did. Though as the series goes on, it becomes more ambiguous whether this is really the case or it's just the tendency of having that sort of godlike power to corrupt dragons in the normal, mundane way that power corrupts
- The Trope Maker. Every spell cast in Call of Cthulhu takes a toll on your mind. And increasing your Mythos knowledge (used to cast spells) permanently lowers your Sanity stat.
- In Exalted, some powerful charms explicitly gain you Limit, taking you closer to a psychotic episode.
- In Magic: The Gathering, this is the case for decks built around the keyword abilities Hellbent, Madness (yeah) and, to a lesser extent, Dredge. Sanity is represented by the cards left in your hand and in your library; an empty hand is unstable, an empty library is when a planeswalker is going to completely lose their mind. Madness allows you to sacrifice short-term sanity to play the card you're discarding cheaply; Hellbent denotes cards that gain an advantage when your hand is empty; and Dredge allows you to affect your long-term sanity to recover things from your graveyard.
- Most cards in the Necronomicon flash card game, with the notable exceptions of Mi-Go Surgery and some low-damage mundane weapons. There are a handful of sanity-restoring cards as well.
- Summoning demons in Sorcerer has a high chance of lowering the summoner's Humanity (which acts as both a Sanity Meter and a Karma Meter in this game).
- Vampire: The Requiem has a few of the most powerful Theban Sorcery spells cost Willpower Dots; once cast, the stat itself goes down. Characters can spend XP to buy it back up to the previous level, but until then they're essentially less able to expend mental and emotional energy. Interestingly, creating a new vampire requires the exact same expenditure on part of the vampire's sire. The setting details some sires who mass embrace too many, too quickly, and become emotional wrecks unable to hold themselves together.
- In Mage: The Awakening, Antinomian Sorcery channels the reality-eroding power of the Abyss and threatens the combined Karma/Sanity Meter with each use, thanks to it linking its practitioners' souls to a gangrenous realm of Things That Cannot Exist instead of the pure Truth of the Supernal Realms.
- Many spells in the Arkham Horror board game have a sanity cost to play, as does the use of Elder Signs to close and seal planar gates; in the Cthulhu Mythos, that sort of thing draws on higher orders of reality than the human mind can comfortably process — like the ones the Eldritch Abominations come from.
- Seems to be a common means of limiting magic use in Fate games.
- In The Dresden Files RPG evocation causes a mandatory hit to the mental stress track, and you can voluntarily take more to boost the skill check, with a potential for consequences or a physical blowback.
- In the Fate conversion of Eclipse Phase asyncs can boost their psi-sleights by risking mental stress, which can lead to mental disorders as consequences. In contrast to the original EP rules, where asyncs risk their hit points and go permanently insane in the process of gaining their powers.
- In Base Raiders magic users are required to either take an obsession aspect or a "taint of magic", and can take Composure stress in order to pass a check that came up short.
- The "Horror Adventures" supplement for Pathfinder includes an optional Sanity Meter system, and some spells cost sanity (in addition to their normal casting cost) if this is used.
- In Stars Without Number psychics can "torch" to temporarily boost their power, which has a 2 in 6 chance of decreasing their Wisdom score, and an equal chance of hurting their Constitution. If their WIS drops low enough they become feral and can Torch with impunity from then on.
- Elminster from Forgotten Realms has been like this since the Spellplague.
- Princess: The Hopeful: Both Lacrima (the Invocation of Tears) and Specchio (the Invocation of Mirrors) have this drawback, though in slightly different ways (the third Twilight Invocation, Tempesta, uses Cast from Hit Points instead).
- Lacrima is all about protecting the ones you care for, no matter the cost to everyone else, and its powers draw heavily on the magics of darkness and despair. For this reason, using many of Lacrima's more powerful charms can cause a Princess to lose Belief.
- Specchio, on the other hand, is fueled by and causes narcissism and solipsism. Every scene where a Princess uses Specchio in any way, she must succeed on a roll or gain one of several detrimental mental conditions.
- In Darkest Dungeon, some otherwise positive abilities lower the morale of the party when used.
- Dawn of War:
- Psykers lose morale when casting lightning (the original effect, still present in the tooltip, was a possibility of killing the psyker).
- In Chaos Rising, you can find Chaos-corrupted wargear, which tends to be much better than standard equipment, but causes the character to fall towards Chaos and gain evil traits and abilities, along with determining the identity of the traitor in the team. Conversely, you can find "penitent" equipment that restricts the character's stats, but reduces their corruption.
- Summon spells in Eternal Darkness gradually cause the player to go insane.
- Mega Man Battle Network: using the Dark Chips in BN4 and BN5 takes away your "sanity" from the invisible sanity gauge. Using them frequently enough will always make Mega Man be in the "dark" state, where he can get access to certain other powerful chips, but disables Soul Unisons and makes him unable to use "holy" chips like Sanctuary or Gun Del Sol. To get out of such a state, you have to do battles without using those chips, and you'll gradually turn better.
- In Level 9's text adventure The Price of Magik, the title is a reference to this trope; casting spells is one way to reduce the player's sanity score, and the more advanced spells require sanity to be below a given threshold before they can be cast.
- In Don't Starve using magic staves consume some amount of the player's sanity, in addition to bringing the staff one step close to breaking.
- The original Bioshock was going to have a variation of this as a mechanic: The player character would become more disfigured and more insane the more Plasmids the player chose to use (much like the Splicers he defends himself against). This was scrapped, presumably because it would've limited the gameplay way too much (even if it would've made sense within the game's lore).
- Dark Mana spells damage the caster's mind (this was what led to Trace going Ax-Crazy), and will eventually kill you if you keep using them.
- Regular magic does this to Basitin who cast it, which is the basis of the Templar conspiracy on their island: Teach them magic and encourage them to destroy themselves.
- In Anachronauts, magic spells destroy whatever they're written on when cast, and most human mages memorize their spells, meaning some brain cells get fried with every casting.
- Emily is/was Cursed with Awesome to be unable to memorize spells, and has to copy them and read them to cast them, retaining her sanity, and prefers utility spells, which means she has to think tactically in a way that tends to elude the other human witches (sanity loss) as well as the elves who trained them (not very creative).
- In Slave Harem In The Labyrinth Of The Other World, mana doubles as your Sanity Meter, and the protagonist becomes suicidally depressed after spending most his mana casting his first spell. He later learns that it is common for dwarven master smiths to commit suicide after their first time creating a magic item, due to the same effect.
- In Adventure Time, the more Simon Petrikov uses the Ice Crown, the more he loses his mind and becomes dependent of it. The key moment where he lost most of his sanity and became the Ice King (or rather an "Evergreen copy") happened when he used it to protect Marceline from a group of radioactive zombies.
- The Alicorn Amulet from My Little Pony: Friendship Is Magic corrupts its user with every use, though the user can return to normal by taking it off (and only by herself).