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Rational magic systems powered by consistent metaphysical laws are cool and useful, but they aren’t the right choice for every project. Whether you want spells to feel hilariously random or your magic is too complex and variable to explain, sometimes it’s more practical to invent individual spells on a case-by-case basis. However, these arbitrary magic systems have some liabilities. Keep them from catching you unaware with these five tips.

1. Avoid Power Creep

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Time and time again, storytellers invent spells because they’ll be convenient for a scene or two, but neglect to consider how many plot holes they’ll create later. When you’re not using robust rules as guardrails to prevent you from inventing a spell that’s too powerful, you have to think through the implications of a spell every time you create one.

That’s because every spell you create could be cast again. Some spells solve an overwhelming number of story problems, and allowing a protagonist to cast those will get you into big trouble.

  • Mind reading makes social conflicts impossible by giving characters lie detection. Almost every story relies on lying at some point.
  • Teleportation means characters can easily escape when they’re in trouble or drop into an enemy’s bedroom and assassinate them while they’re sleeping.
  • Large-scale destruction means characters can win fights even when they’re greatly outmatched or outnumbered.

You don’t want to write yourself into a corner by making threats too easy for protagonists to deal with. Plus, even when a powerful spell doesn’t turn solving problems into a cakewalk, it can make conflicts less interesting. If your character can shoot fireballs to a location one hundred feet away, they’ll keep their distance instead of getting into close quarters with enemies.

So when you’re choosing what spell your protagonist might cast, look for the weakest power that could do the trick. Instead of dispersing a whole storm so a ship at sea won’t sink, maybe redirect the wind around the ship or slow it down a little. Your magic doesn’t have to be powerful to feel wondrous; it’s magic. You just have to describe it in a way that makes it feel wondrous rather than mundane.

When characters other than the hero cast magic to establish the world or set the atmosphere, avoid making it look like something your protagonist could replicate. Maybe no one knows how the master wizard is able to summon a stairway of light.

2. Set Flexible Limits

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Limits are helpful with every type of magic system, but some kinds of limits are especially useful for arbitrary systems. With rational magic systems, limits are often created by examining the basic operation of magic to find ways that spellcasting could be prevented. For instance, if a wizard needs a staff, that staff could be taken away. If casters need to do martial arts moves, tying them up could keep them from using their abilities.

Those limits are still useful in an arbitrary system. However, arbitrary systems often include so many different magical effects that the issue is not whether characters can cast magic at all, but which spells they can use. If your protagonist previously dispersed a storm to save a ship, and now the big bad is using a storm to threaten characters on a mountain, you need to ban that storm dispersal spell. Otherwise, it will be too easy to rescue those characters.

Storytellers often try to fix this by explaining how this new storm is different. If the protagonist is taking on a challenge that is obviously more difficult than what they did previously, that can work okay. But, in many cases, declaring that a previous spell won’t work in the current situation feels contrived. The more times you use it, the worse it will sound.

Instead, you can create a few rules for your magic system that makes replicating a previous spell harder.

  • If every spell uses up a rare and expensive ingredient, then what spells a mage can cast will depend on what they have on hand.
  • Your protagonist might have to cast what they’ll need in advance. Anytime they get in a fix that’s time sensitive, they’ll have a limited list of spells to work from.
  • Fickle spirits that grant magic can also be used to explain why a spell can’t be repeated. Maybe a different spirit is available every time, or the spirits get bored if they’re asked to repeat the same favor.

Using these kinds of limits, you’ll be able to declare what spells your protagonists have access to for each conflict.

Another common way to limit magic is to give it a significant cost. Spells that make the caster sick or require them to destroy something valuable can be fun additions to a story. However, these costs don’t really fix overpowered spells. In many stories, casting a spell is a matter of life or death. If the whole city is about to go up in smoke, even severe consequences are no longer a deterrent.

3. Keep Track of Everything

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As stated above, the spells you invent don’t evaporate afterward; they become a permanent part of your magic system. Once you’ve established that a witch can smoke a catnip joint to summon a horde of cats, your readers will know they can do that for the rest of the story. If you only wanted to create a swarm of cats for one scene, it’ll be easy to forget the catnip spell when it comes time to defeat the plague rats. But while you may have forgotten it, not all your readers will have.

This is why I recommend rigorous note-taking. Every time a character does magic, put it on a big list. Write down its effect and any methods the character uses to get that effect, such as speaking incantations or waving their wand. Be as specific as you can. Once your witch summoned all those cats, did they do her bidding or did they act like they were stopping by of their own free will? You might also want to include the chapter title to make it easier to go back and read the scene the spell is in.

Keeping track of spells will help you in several ways.

  • You can only fix plot holes if you know they’re there. When your characters get in a fix, you can run through your list to see if there are any spells that would get them out of it too easily. Then you can look for a way to change the spell or the situation so that it’s no longer true.
  • You can identify spells to reuse. While new spells have more novelty, reusing a spell once in a while will make your magic system feel more real. Lower-stakes conflicts are the best time to reuse a spell you invented from an earlier scene. If your notes aren’t detailed, read through the scenes where it was last used to make sure it works the same way.
  • You’ll have a great place to start if you want to tweak the rules of your system. For instance, by looking at your spell list, you might find that most spells involve speaking incantations, but two don’t. To give your spellcasting a more consistent feel, you might add incantations to those two spells. You might even find that converting your spell list to a rational system isn’t as hard as you thought.

4. Introduce Spells Before They Matter

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If your magic system is rational enough that readers can extrapolate what magic can do even if they’ve never seen it, then you have built-in foreshadowing. That means if your protagonist pulls out a badass new spell at the climax, that will probably qualify as a clever deduction turning point, creating a satisfying ending. But if the protagonist defeats the villain with an unheard of spell in an arbitrary system, it’s a deus ex machina. No one likes those.

This doesn’t mean that the protagonist can’t use magic to defeat the big bad or surmount other obstacles, but it takes some setup. If you’re discovery writing, you’ll need to go back and make revisions to earlier scenes.

Generally, your audience needs to know about a spell and its effects before a protagonist uses it to win a conflict. If your protagonist uses a spell in an early scene, they can pull out that same spell later to defeat a villain. You’ll still need a turning point, of course, but the spell itself won’t feel contrived. It may seem like this takes the fun out of it, but you can spice things up so that using a known spell doesn’t feel perfunctory.

  • Surprise the audience with which spell you use. It’s really satisfying to watch protagonists use an insignificant spell in a clever way to create a big effect. Perhaps the character worked hard on a basic spell that seemed useless until the big moment, creating a prior achievement turning point.
  • Have the character cast the spell successfully for the first time. Introduce a really cool spell that the character hasn’t been able to cast. Be specific about what’s holding them back; you’ll need that to justify why they have a breakthrough at a critical moment. Maybe the spell requires a strong sense of purpose, and they don’t have that until it’s up to them to save the day.
  • Make the protagonist piece together clues to figure out how to cast it. Perhaps a legendary mage opened a magical gateway centuries ago, and no one knows how they did it or how to close it. Plant clues so that your protagonist can figure it out in the nick of time. It’ll be easier to make this reveal click together if you narrow down your spellcasting options. Perhaps the gateway spell is in an ancient book, but it hasn’t worked for anyone who’s tried it. Then the protagonist can figure out the one thing that has to be done differently for it to work.

If a protagonist isn’t solving an important problem, you don’t need to be as cautious. Foreshadowing isn’t required before they cast a spell to make the fruit in their breakfast ripen or craft a mild protective charm to trade for goods at the market.

5. Theme It

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I know I say this every time I mention worldbuilding. It’s still true, especially for an arbitrary magic system. Rational magic feels believable not only because the audience understands how it works but also because the rules force all the spellwork to be somewhat consistent. When your magic system is a collection of spells and effects you created when you needed one, you’re likely to end up with an eclectic set. To your audience, some of your spells may feel, well, arbitrary. You want spells to feel natural, not like you made them up on the spot. Even if you did.

Spells are likely to feel contrived rather than believable when they’re different from what you’ve previously established. If the first three spells you pull out are all elemental spells and then your character waves their wand and a smashed piano snaps back into perfect condition, that will draw scrutiny.

Using a theme for your magic system helps protect against this.

  • What atmosphere do you want to create? Should magic be ancient and wondrous, a dark temptation, or wacky high jinks?
  • What is the origin of magic? Does it come from the blood of the old gods, calling their descendants to heroism? Is it cosmic energy that’s only perceived by those who are psychically sensitive?
  • How should magic look aesthetically? Do you see your mages doing fluid dance moves, burying their noses in books, or dropping an eye of newt into a bubbling cauldron?

Using an arbitrary system means you have complete freedom in describing your magic. Unlike in a rational system, in an arbitrary system you don’t have to justify why characters are chanting or holding hands in a circle. If you’re not using that freedom to make spells feel mystical, dangerous, hilarious, or something else, what’s the point?

While you may want to keep your options open, you can also narrow down the breadth of what spells can do. That will help you create a matching set, adding credibility. For more on this, see Creating a Magic System for Superpowers.

I’ve been hard on arbitrary systems because they’re more likely to be used by storytellers who haven’t thought their magic through. Examples abound of stories where the protagonist pulled out a godlike spell only to forget about it in the next scene. However, in the hands of the right storyteller, arbitrary magic systems can be used to great effect.

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