Multiplication is Magic
(Drawing by Evlyn M)
The only people who have ever done magic have been executed by the primary religion or are in hiding and the player characters probably don’t even know about any of them except one or two executed wizards. (No magic classes are available to players.)
If a player character ever starts to use multiplication, they’ll feel an intense surge that resembles drinking too much coffee. If they actually finish a multiplication operation, a spell will be cast.
Now everyone in the party is a wizard, if they’re foolish enough.
Division teleports you and everyone nearby to the Underdark/Veins of the Earth. Division down below casts spells there. Multiplication has no effect in the depths.
Doing square roots collapses the mathematician’s lungs.
Drawing perfect geometric shapes causes supernatural mutations.
Other kinds of math have no effect whatsoever.
Correctly multiplying a negative number by anything grants a wish and sets the apocalypse in motion.
Example: a player character says “4 torches each. There are three of us, so that’s 12 torches.” Now, use a list of spells and randomly choose a spell. I’d probably just use the 12th entry of the wizard section from the short concise list of traditional spells. This means the character would, without warning, cast a 5x10ft cone of flames in whatever direction their palms happen to be exposed to.
No spell has any a daily limitation. Spells aren’t forgotten. You may wish to make spells random rather than tied to specific multiplication results if you don’t want humongous amounts of magic. Don’t worry though, they’ll destroy the world soon enough and it won’t matter anymore.
Players know none of this until it happens. Almost no one else does either, though religious leaders believe that all math which is harder than addition or subtraction is VERY WICKED and TERRIFYING.
Bonus! I’ve been asked about how taxation and buying things in bulk could be possible. The real answer: “figure it out.” For fun though, here’s some possibilities.
Taxes: Option 1: The King owns everything. He may use what he pleases. Option 2: Everyone owes 50 gold or all of their food, whichever is available.
Bulk Purchases: Option 1: A torch’s price is 5 copper. I want 100 torches. Okay, 5+5+5+5+5+5+5+5+… Option 2: That’s a lot of torches. I’ll give you a cow for those.
Further discussion, including lots of ways to do complex math without casting magic.