Sometimes, even heroes like to live on the edge.
A Random Effect Spell is just that—a spell or item in a video game that pulls from a pool of random effects, instead of doing the same thing every time. Often (but not always) granted as a late-game spell or ability, or difficult to come by, the Random Effect Spell's effects can run the gauntlet from "really cool" to "really lame". How big the effect pool is can depend on the item or spell, though frequently, at least one or two of them will be a duplicate of an already-existing spell or ability (if not all of them). Frequently, at least one possible effect of the spell will be completely amazing (such as dealing massive damage to all enemies), but another will be completely horrible (such as cutting all your characters' health in half). Also frequent is one option which does absolutely nothing. Whether the risk is worth it generally depends on the ratio of good effects to bad ones in the pool, the cost of the ability, and how many (and how strong) the enemies are you face.
Due to its random nature, it is frequently Awesome, but Impractical and a Useless Useful Spell, but not always. If there are enough good effects, they can easily be a Lethal Joke Item or spell. If the randomness results from the spell being miscast, then that's Magic Misfire. Compare Randomized Damage Attack where it's only the damage that is random, but the effect stays the same - although they may overlap if one effect yield greater damage than the other.
Troper General's Note: Many spells or items in video games (and other games) have an element of randomness to them to keep things unpredictable—such as the additional damage from one spell being randomized, or one spell possibly being able to bestow a number of Standard Status Effects. However, for the purposes of this trope, we are primarily concerned about examples whose primary purpose or effect is the randomness.
See also the Super TropeUnpredictable Results.
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Earthbound has Paula's Pray comand, which does a random assortment of things. It's free and can heal people and give status effects to enemies, but it can also give status effects to you—so outside of the now-infamous final battle, you're generally better off sticking to Paula's Black Magician Girl skills.
"Metronome" can use any other move in the game. At least one spin-off game had "Metronome mode" fights, where you and your opponent had two Pokemon knowing nothing but Metronome, and saw who got luckiest first.
"Assist" picks a random move known by any fellow party member, making it slightly less unpredictable than Metronome.
Delibird's "Present" move either will either inflict light damage, moderate damage, or moderate healing on the opponent.
Sleep Talk picks and uses another one of the Pokemon's skills at random while it sleeps, making it most predictable but still random of all.
Chance in the various incarnations of the Dragon Quest franchise.
The Mystery items in the Paper Mario franchise, and, to a lesser extent, the Kooky Cookies. Almost everything Mystery gives you is good, and the Cookies are one of the only ways (outside of the rare Repel Gel) to turn see-through, so they definitely have Lethal Joke Item potential.
The rare Random Hearts equipment in Opoona adds random effects to all of your normal attacks, as dictated by a number.
The Fool Arcana Chance in Persona 4 becomes any one of the other Arcana cards when it appears, adopting their effects.
Teddie also gets a random effect spell in the remake, going by the name "Kamui Miracle" which would cast a random status ailment on your entire party or the enemies being faced, knock everyone down, or restore everyone's SP and HP stores.
'Gambler' types (Setzer, Selphie, Cait Sith, Lady Luck) have a variant on this that allow them to use a slot machine or dice to pull out a nearly random effect, but odds are not entirely random, due to a slightly skill or luck based timing on the slots.
Final Fantasy IV has a character named Tellah, who is an old sage who has forgotten most of his spells. When you first recruit him, he only has a smattering of low-level spells, but has an ability named Recall, which allows him to cast a random spell from his formerly expansive repertoire with no MP cost. This can be anything from a low first level ability to some of the late game powerful effects, and unlike the normal versions of said spells, Tellah can target multiple enemies with spells cast this way, even if the normal versions of the said spells can only hit 1 target.
In FFV, the Hunter can summon a random animal, and the Geomancer can effect random effects based on the environment. The summoner also learns a command that allows them to call a random summon that they've learned, without using MP. Finally in the GBA and iOS/Android version, all four new classes get one command that lets them do something (seemingly) randomly - the Cannoneer gets Open Fire which randomly uses one out of four possible attacks; the Gladiator gets Finisher which either inflicts a Critical Hit, deals 9999 damage with element that matches the character using it, or does nothing; the Necromancer gets Oath which again randomly uses one out of four possible attacks, and the Oracle gets Predict, which seemingly randomly picks an attack that could target either allies or enemies (or both), does random damage, and randomly inflicts some sort of status based on the move randomly chosen. note Except that it's not random at all but based on the last digit of the caster's HP and MP. Funny how the most random looking skill is actually not random at all, right?
Final Fantasy VI has the Magicite item that calls an Esper to the battle, but which one appears is purely random, ranging from the useless Siren to the lethal damaging Bahamut. It's useful early in the game when Espers aren't available to you yet or you haven't gotten the stronger ones yet. The item version of summoning Espers doesn't cost MP to use but you can be completely screwed if the wrong one appears. Also, Mog's special skill, Dance, lets him perform one out of four random attacks based on the dance chosen, and Gau's special skill lets him use either a physical attack or a special command/spell based on a monster within the game, amongst other effects.
Lunar Magic on Legend of Mana/Seiken Densetsu 2, it may cure you at full health or decide to turn you into a bunny, any random effect on both your party and your oponents
Bahamut Lagoon: The Mini-Devils' Dances are entirely unpredictable; the pool of effects can be changed based on the dragon associated with them, but there's rarely more than about a 60% chance of getting a positive result. Results can range from various element attacks with varying ranges, healing your party, healing the enemies, healing everyone, putting targets to sleep or poisoning them, etc.
Acrobats in Dokapon Kingdom can get the "????" skill. It changes to a random skill every time a new battle starts.
Falitza's "Press Fortune" ability in The Reconstruction. Whether or not it even causes a good or bad effect to occur is random. Its effects aren't too varied, though — just full-party buffs or debuffs. It can be upgraded to have four new effects, though — increasing/decreasing the Rush meter, and advancing the skill chain by 8/breaking it.
A number of Viviosaurs in Fossil Fighters Champions have random effect spells. Coelanth has an ability that randomly inflicts any status effect, and Archaeo has one that's just random, period.
In the PS3 remake of Tales of Vesperia, almost all of the arts and spells used by Patty Fleur are random in regards to what they'll do, though the odds of something good or bad happening can be tweaked by equipping certain skills. The most random of all is her Critical Moment spell, which has dozens of possible effects that make good or bad things happen to the party, the enemies, or both.
The Sage job tree in Ragnarok Online has the spell Abracadabra. It randomly casts/executes any skill or spell available to any job, as well as some unusable through any other means. The most sought after effect summons a random boss monster, which is then typically killed for rare loot.
In Kingdom Hearts 3D, the Juggle Pup and R&R Seal's Lucky Dice link attack can cause all sorts of randomness to happen to either you or your foes and for better or for worse. Not something you want to try out in a boss fight, as there's a chance for it to completely restore the HP of all enemies present. All of their basic attacks are random, as well, their normal dice attacking allies or enemies based on what side lands face up, and their ball/melon tossing attacks doing either no damage, decent damage, or high damage based on the color of ball/melon used.
There's also the always beneficial Break Time command, that restores either your HP, link gauge, or drop gauge at random. As the name might imply, it's not really meant to be used in combat, as it leaves you wide open to abuse for a lengthy period of time (Though using it in midair offers a good degree of safety from many enemies).
A number of dream eaters get this treatment in the Flick Rush minigame, with two or more of their attacks being consolidated into a single card that uses one of them at random when it's played. The Ducky Goose gets the most randomness of all, having three different possible attacks that can be chosen, and said attacks having a randomized length and/or random number of weapons produced.
The Secret Box items in Mario & Luigi: Dream Team are this, being a random effect ITEM. You use a non descript box with a ? mark on, something random happens in battle. Could be good (healing both bros, doing huge damage to enemies), could be bad (healing enemies) or it could be plain suicide (wipes out everything on the field including your playable characters in a One-Hit Kill).
Mr. Game and Watch has his "Judge" ability. A random number appears over his head, tied to corresponding effects. It can do everything to setting people on fire, freezing them, and (if you get lucky) causing an instant KO, and if unlucky, he will hurt himself.
There's also the Poke Balls and Assist Trophies, who summon a random character from their individual lists to do different things. Two Pokemon, Clefairy and Togepi, even do random things within the random item! (In the first game, Clefairy copies another Poke Ball Pokemon. In the other games, it's a smaller pool of unique effects.)
The Surprise Device in Water Warfare does several effects, ranging from the beneficial-to-all (everybody gets healed and has their water refilled!) to the bad-for-all (everyone gets rained on and takes damage! Ack!) to the bizarre (Pan On The Head, it gets dark for some reason). And of course, the standard "nothing at all happens" effect.
In Peggle, Warren's special attack spins a wheel that randomly gives you an Extra Ball, Magic Hat, Triple Score, or a random powerup from another character.
In the case of Ancient Domains of Mystery, a sip from a pool could cause one of many effects, ranging from a free wish, through gaining or losing stat points or intrinsics, or even having a small frog pop out and give female characters a small golden ball and a "Frog Quest".
Dungeon Crawl features several different examples. One such is the Wand of Random Effects, which is Exactly What It Says on the Tin. You might launch a Fireball spell at that ravening ghoul... or you might end up healing it instead.
The Scroll of Random Uselessness is another such example. Only a few possible outcomes will do anything, and even those effects are extremely minor — you might summon a few butterflies, or the scroll might just reassemble itself so you can use it again.
You can drink out of sparkling fountains to gain the effects of various potions. This is generally a last resort, because while it's nice to get a free healing potion out of it, you might also wind up with a potion of poison or mutation.
Various weapons can have the "chaos" brand, which means the weapon will do random things upon hitting the opponent — burning them, freezing them, and poisoning them are but a few of the many possibilities. Certain unique monsters, like the gnoll Crazy Yiuf, always come with chaotic weapons.
Finally, certain deities approve of their followers taking risks with Random Effect Spells. Nemelex Xobeh, the trickster god, will grant you decks of cards which will do various things when drawn. With the proper strategy, though, said decks can provide a wide range of useful abilities.
In Final Fantasy Fables: Chocobo's Dungeon, the final ability for the Scholar job is "Truth", which casts a random spell when used. In the Nintendo DS remake, this is now the Scholar's first ability; the new final ability is a variant narrowed to the most powerful spells only.
Elements Flash card based game has several examples of this type, mostly in the Entropy element:
Mutation causes the the target creature to die, turn into a mutant, or turn into a different, random creature with a random power. Additionally, the Entropy Wild Elf has an ability that does exactly the same thing.
In addition to its normal effect of returning a creature to the top of the deck, Reverse Time can, when used on a Zombie, turn them into a random creature.
There's also Chaos Seed (a spell whose effect is randomly picked out of 11 different possibilities), Pandemonium (a spell whose effect is randomly picked out of all the available effects in the game), Skeleton (a card that can turn into a random creature using the spell Reverse Time), and Fate Egg (a card that turns into a random creature).
Prismatic Spray, for example, hits each target with only one or two of its seven different rays. Depending on which rays hit, the enemy can suffer varying damage, die, go insane, get petrified or get ejected into a different plane.
Some items, like Wand of Wonder and Deck of Many Things.
Nahal's Reckless Dweomer requires you to roll on a table with 100 diverse entries for its spell effect. Of course, many longer lists can be found on the internet. The associated class, the Wild Mage, has a 5% chance of causing an effect from this table whenever casting a normal spell.
Magic: The Gathering Online's "Vanguard" has several Vanguard avatars which pull random effects like these. Most prominently, Momir Vig allows you to pay X mana to make a copy of a random creature that also costs X mana, spawning an entire alternative format called Momir Basic, where players build a deck using only mana sources and a Momir Vig avatar and battle with randomized creatures from all over Magic. Jhoira of the Ghitu has a similar effect for instants and sorceries; likewise with Stonehewer Giant and equipment.
Mind's Desire casts a random spell from your deck for free, and multiplies itself for how many spells had been played previously. Decks based around using it to cast lots spells that in turn multiply themselves or free-cast even more spells can get very silly - and very deadly.
Strategy, Schmategy has you roll a six-sided die to determine which of five totally unrelated abilities you'll get when you cast it. To up the ante, one of the options is "Roll the die two more times."
Kid Icarus: Uprising has the Random power, which causes any one of the other powers to be used when activated. God help you if it chooses Spite though.
There's also the Random Effect power, which restricts the possibilities to adding one of the Standard Status Effects to your attacks.
Several spells of the 'Chaos' domain fall under this - unsurprisingly. The 'Mutation' spell applies a random number of random effects to the affected unit, and can do anything from turning them into a greatly-empowered hero, to turning them into a withered, cannibalistic husk.
Greater yet is the 'Wonder' spell, which can have a vast array of effects - including nearly the entire archive of general spells, as well as a number of unique effects you can ONLY get from it: Creating penguins, causing giant mushrooms to grow, opening the very Gates of Hell... or turning you into a baboon.
Baldur's Gate II: Throne of Bhall had the Wild Mage whose specialty, Wild Magic spells, could result in any one of a hundred random effects. Some were useful, such as giving the party bonuses, extra spells, or the mage actually succeeding at casting the spell they intended to. Or it could do useless things, like opening and closing all nearby doors, or a few absolutely hilarious things.
Micropose's Magic: The Gathering video game had a set of Astral cards (one for each colour) that did exactly this.
A recurring artifact called Wabbajack in The Elder Scrolls series turns enemies into random monsters, ranging from simple rats to minibosses. In The Elder Scrolls V: Skyrim it is a Lethal Joke Item and minor Disc One Nuke. It's very easy to obtain from the start of the game, its most common effect is an elemental blast that is more powerful than anything a low-level character could manage, its rarer effects are mostly positive things for the player like transforming enemies into harmless bunnies, and the worst thing it can do (turning the enemy into a powerful demon) can be undone just by hitting them with the staff a second time.
In Akalabeth (the first game in the Ultima series), the magic amulet had a random function that could cost you half of your hit points or turn you into a Lizard Man or a Toad.
The Jester Wand and the Archmage spell "Wonder" in Egoboo have a variety of random effects.
In Dofus and Wakfu, the Ecaflip (gambler) class is based around this type of spell. In Dofus, they have attack spells that have a chance of healing opponents instead, attacks that wait a random number of turns before actually taking effect, and a spell that buffs everyone on the battlefield - friend and foe alike - with a random effect, just to name a few. Wakfu has similar attack spells, plus a passive skill that gives the character and/or the enemies a random buff or debuff each and every turn of combat.
Clonk has the aptly-named "Random Spell" spell, which casts any of the spells loaded as a definition, even ones not normally available in the scenario. Even in a scenario without any unavailable loaded spells, it's still not useless though obviously luck-based, since it has a very low energy cost.
Exciteshrooms in the Monster Hunter series can take the effect of a random consumable item when they're used, though there's also a chance that they'll do nothing at all.
In ''Culdcept, the Protean Ring and Baldanders are an item and a creature (respectively) with the same effect: They turn into a random creature during battle. You could become anything from the meager Giant Rat to the insane Bundle Gear/Gearion. It also only lasts for one battle.
Non-Video Game Examples
Anime and Manga
Joey/Jonouchi from Yu-Gi-Oh! uses a deck with many cards that depend heavily on luck. For example, the Time Wizard can destroy all the monsters on either side of the field depending on its roulette result, dice-themed cards can multiply one of his monster's attack values or divide an enemy monster's attack values, and another card makes an enemy monster randomly attack a monster on either side of the field. Although Joey's luck often works in his favor, it has also backfired on him quite severely on more than one occasion.
In the Pokémon anime, the Togepi that spent time in Misty/Kasumi's care secretly knew one ability that seemed to show up at the most coincidental moments. The ability was Metronome, already mentioned above in Eastern RPGs. An early episode showed a colony of Clefairies whose have the Metronome as well.
Nico's Staff of One in Runaways can cast any spell she can imagine, but she can't use the same spell twice without running into this trope. The first time she tries, a flock of pelicans flies out of her staff. She was trying to freeze the room. Later, she tries to fumigate a swarm of bees (forgetting she'd already done this off panel) and winds up teleporting several miles into the local desert.
The My Little Pony: Friendship Is Magic fanfic Study Break has a species of apple that only grows in areas with high levels of background magic. When Twilight unwittingly eats one, it causes random effects everytime she sneezes (beginning moments after she finishes it), including shrinking, invisibility, glowing, floating and making her look "like a badly used paintbrush", and that's just within a few hours - she expects the effects to continue for two or three days.
In Goblins, the Shield of Wonder causes a random magical effect when struck. Among the things it's done: turned someone's sword into an entanglement effect, came to life and ate the wielder, caused bladelike force fields to appear all over, and created Baleful Polymorph landmines.
Quentyn's sword Wildcard from Tales of the Questor. Possibly based on the game trope. It does, however, always come with something useful for whatever crisis he is in, if he is in one. (Out of one and trying to test it, he may find it announcing what time it is — and getting it wrong.)
Web Comic example: Homestuck has the Fluorite Octet, an enchanted set of eight-sided dice wielded by Vriska Serket, which when rolled result in one of 16,777,216 unique effects, ranging from the devastatingly powerful to the completely useless.