There's an innate design tension which RPG magic systems must contend with.
Magic must be more powerful than enacting the equivalent effect by mundane means, otherwise it's not fun and not fantastic. But if it's powerful and its uses are limitless (or at least non-rate-limited), then you become committed to an exceptionally high-magic setting.
If it takes ten seconds to give yourself a magical boon which lasts for ten minutes, the default player behaviour and the reasonable in-world behaviour will be to cast the spell every ten minutes... Unless they have a specific reason not to. That's almost always 'resource management'. In almost ever fantasy TTRPG, magic (whether spells, items, rituals, innate powers, etc) will be powerful and limited, i.e., a valuable resource; something to be conserved for when it will matter. Therefore, players will want to know the exact effect of some magic they might use, so that they can make an informed choice.
And this creates a design tension. How predictable to players is the magic their characters are harnessing?
The predictability predicament
Assuming you want magic to be able to do a huge range of things (it's magic), to get breadth of coverage, you either end up with these huge, minutely-detailed lists of magic effects and parameters (almost certainly player-facing), or you have a fuzzy system where the GM instantiates vague wordings at runtime (possibly via what amounts to negotiation with the player). These game design choices have different costs in terms of usability and satisfaction.
Most big, popular fantasy TTRPGs like D&D go with the huge, minutely-detailed lists of magic effects option. This arguably results in greater player agency, in the sense of being able to make good and meaningful decisions about resource allocation, but it constrains magic to the big official list (and if it's not exceptionally well-designed, you end up with weird codified corner cases and interactions). You also need to know the system really well to be effective as a player or a GM. Finally, there's a risk that presenting in exhaustive detail what is possible strongly implies that anything else isn't possible. You get cases like D&D 5e, where many destructive spells officially don't damage objects because a small subset of them specify that they do.
The fuzzy system option is attractive because it makes magic more 'mysterious' and wild, and thus fantastical. But it can be hard for a GM, especially one who isn't already operating in a "rulings not rules" tradition. But it can also be hard for players to come to terms with, unless either (a) it's the case in-world that a mage never knows exactly what they're going to get when they do some magic (which is flavourful, but risks a poor play experience), or (b) the GM and player together determine what happens when the magic is used. The latter might sound good (an opportunity for collaborative buy-in) or bad ('Mother May I' gameplay). Some GMs are surprised to find that many players hate 'collaborative storytelling exercises' spilling over into their roleplaying games. Either way, it doesn't actually resolve the tension: it's still in the player's best interests to stop and discuss the possible magical effect before they can make an informed choice whether to use their magic.
Variants of the fuzzy system where e.g. the GM has to follow certain rules to come up with a power-level-appropriate instantiation of a spell at run-time are close to being the worst of both worlds: mechanically fiddly but without giving a player enough information to make decisions with, and you just end up building that minutely-detailed canonical list slowly over time.
Resolving the tension
I consider this an open problem. As I've suggested before, I think the best practises are to
- Pick a path and stick with it, rather than mixing up how rigid and predictable the magic system is in different use-cases within the same game; and
- Have an extremely strong, well-thought out metaphysics which either underpins the mechanical design for the minutely-detailed lists option or informs what 'should be the case' for the fuzzy system option.
Food for thought
Here are some questions your magic system must be robust enough to answer, consistently:
If you turn flour to silver dust and mix it into acid, changing it to silver nitrate and creating heat, what happens to the silver nitrate if the spell wears off? What happens to the heat? If the compound is left over, can you reduce it to get silver? Does the resulting silver still count as being magical?
If you turn a pile of stones into loaves of bread and spend a week eating them, what happens to your tissue, the partially-digested bread, and the waste, if the magic ends? Do you die instantly? Does the energy you got from eating disappear?
If you turn a stone into a cat, and that cat eats and sweats and breathes makes waste, then the magic is dispelled, what happens to the bits of world (food and air and water) that got incorporated into the cat's body? Are they in the stone? Do they turn to stone? Can they be turned back? What happens to the cat's sweat and waste and loose hairs and skin particles out in the world? Do they turn to stone? What if the cat was a kitten, and it grew much bigger and older before the spell ended?
Or if the original subject was a mouse rather than a stone, what happens to the cells that changed in its body? Was there a 'mapping' from one mouse cell to multiple cat cells, so that injuries and shed cells and replaced cells carry back over? Do microbial diseases carry back over? Does cancer?
Are all your answers consistent so far?
What happens if you turn someone into a statue and snap their arm off? What about if you turn someone into steel and then temper or anneal the metal? Or take a mould of their body, turn them into ice, melt them, and then refreeze the water in the mould?
What about if you turn a piece of metal into a piece of wood and burn it inside a big tank of air? Does the carbon dioxide turn into a metal oxide? In general, if you heat something up while it's magically transformed and then turn it back, is it still hot? What if you turned something really small into something really big and then it just got slightly warm by soaking up a million times more sunlight than the tiny thing would have?
If you expand a mouse to be the size of a horse and it eats a bucket of oats, what happens to the oats if it turns back? Does it matter whether they've been digested?
If you expand a mouse to be the size of a horse inside a mouse-sized terrarium that a mouse couldn't possibly break but a horse easily could, does the spell work? Does the answer change if the terrarium is nearly horse-sized? What about if you try it on a mouse that's inside a crack in solid rock? Is this meaningfully different from trapping a mouse under a piece of heavy cloth and then expanding it to the size of a horse?
If you cast a spell on a subject, then another one that seems to have the inverse effect (flesh to rock then rock to flesh), do you get back the exact original? If so, why? If not, what happens if only the first spell ends or is dispelled? What if you cast the first spell, then make some orthogonal change to the subject, then cast the second?
What in-universe mechanism decides whether an object counts as meeting the targeting requirements of a spell? Does a spell that works on chairs work on a one-legged stool if the caster doesn't think a one-legged stool is a chair?
Do any of the answers to these questions change depending on whether it's a temporary effect, a dispelled permanent effect, a magically reversed effect, an intrinsic ability of a magical creature, the effect of an item, etc?
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