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These are the external factors needed to make a spell or ritual work. They fall under five main categories.
- The right time - when the moon is full, the summer solstice, an eclipse.
- The right place - somewhere mystically significant, such as Stonehenge
- The right target - spells that only work on werewolves, or people wearing red.
- The right caster - spells that can only be cast by someone genuinely happy, or on the brink of death.
- The right condition of negation - spells that only work if there is a way to undo them.
They range from difficult to completely impossible for the caster to control. Persuading the target to wear red is fairly easy; making the sun set on command is beyond most wizard's powers.
Spells that only work at the right time serve to control the pace of a plot. If the Big Bad can only cast the ultimate spell of world domination on one specific night, that both gives the heroes a deadline, and explains why the Big Bad didn't conquer the world ages ago. Spells that only work in the right place lead to battles for control of the key sites.
Inherent gifts are another common form of prerequisite; the spells only work for people born with the right talent.
- The Contractors from Darker Than Black, who are required to perform Remunerations whenever they use their powers, lest they suffer from Power Incontinence. These Remunerations range from smoking to eating boiled eggs to writing poetry to drinking the blood of children, and a Contractor doesn't exactly get much choice in which one comes with their powers.
- A number of spells in Mahou Sensei Negima require that the magic of The World Tree be active in order to work, such as the spell the the festival arc's Big Bad attempts to use to break The Masquerade.
- In Yu-Gi-Oh, Atem's power can only be activated through games. If his opponent loses the Shadow Game, he is free to dole out magical punishment.
- In Harry Potter, the brewing of Polyjuice Potion involved particular parts of a lunar cycle.
- Also, it's stated that a certain mindset is needed for certain spells, particularly hatred for the Cruciatus and Killing Curses.
- And the Patronus charm, where the caster must be thinking of the happiest thought they can muster, which, given the circumstances of using the charm (being attacked by an Emotion Eater), is no easy task.
- In The Magicians, the effectiveness of spells vary depending on numerous factors (phase of the moon, the weather, etc) that students at Brakebills have to memorise. The final year at Brakebills South is dedicated to ensuring that the graduating students can act on these factors without even thinking about it.
- Also, magic has to be practiced with a relaxed and stable mind, otherwise a particularly spectacular Magic Misfire will occur.
- Dresden Files: Murphy asks Harry why he just doesn't do the "pocket full of sunshine" trick on the Red Court vamps (sometime after Stormfront I believe). He states it turns out you have to be genuinely happy to make the spell work.
- Jonathan Strange and Mr. Norrell: Many spells only work while the "King's Roads" are active.
- This is the main limitation of magic in Rick Cook's Wiz Biz series. The more powerful the spell is, the more prerequisite it has and the more likely it is to fail or do something completely different if the prerequisites are not filled.
- The 'negation' angle comes up in the Discworld novel Sourcery. A wizard seeks to escape Death by sealing his soul inside his wizard's staff. Death tells him no deal, unless there's a condition that causes his soul to be released (because nothing escapes Death.) The wizard offers to let his soul be freed if his son willingly tosses aside his staff. Death accepts.
- This is the only thing that keeps the universe of Buffy the Vampire Slayer from ending, since all the BigBads that want to destroy the world have to wait their turn and go to Sunnydale, where they have to face the Slayer.
- The cantrips of Changeling: The Dreaming had bunks, little obligations to the Dreaming that would make using a certain ability easier. Want to fly? Burn an eagle's feather, or tap your foot three times. Want to talk to rocks? Paint a little sigil on them. The more complex the bunk, the more likely the cantrip is to succeed.
- The successor game, Changeling: The Lost, does the same thing; each Contract has a specific Catch. By performing the Catch, the changeling avoids spending Glamour to activate the Contract. These Contracts range in applicability from "You must be fighting multiple foes, bare-handed, by yourself" to "You must use this power within five minutes of noon" to "Your target must be wearing or touching gold."
- Several cursed (or in this case unreliable) items from D20 srd have prerequisites that would render then practically useless, others just need to be used in the dark or by a certain gender.
- There are a number of spells in Magic the Gathering that can only target creatures of a certain color; more commonly, creatures will have "protection from [color]," preventing them from being harmed by creatures or spells of that color. Finally, there are some spells that can target anything-but-color; this is mostly Black being unable to destroy other Black creatures, possibly symbolizing how Black can be its own worst enemy.
- Bloodbending in Avatar: The Last Airbender requires that there be a full moon. Explained by the moon being the power source for waterbenders, of which bloodbending is a complicated form.
- On a variant, there is ten-fold amplification of firebending that allows for grand conquest only happens once every hundred years - Sozin's Comet. Whether conditions exist that would amplify air or earthbending (and why a comet would amplify fire and not earth or water) is open to speculation.
- On a similar note, the various forms of Bending and the way they affect their respective cultures tend to breed different ways of looking at the world; in order to master any given form of Bending, Aang has to learn to think like its practitioners. Earth gives him particular trouble because the forceful, head-on approach of the Earthbenders is the polar opposite of the Zen, understand-all-angles approach he was raised with as an Airbender.
- In a similar vein, Korra in the sequel has already mastered three of the four elements but has a mental block when it comes to Air because she is a Hot-Blooded person who likes to face challenges head-on. She also cannot tap into the Avatar state because of her lack of spirituality.