The magic works like this:
- It must not be performed in public, meaning with more than a dozen observers (regardless of the means). It will fail, spectacularly
- It must not be performed in private, meaning that if you do it alone, it will also fail fatally
- The optimal setting for it to work is in a group of less than or equal 13 - the more participants the stronger it is
- The caster will have to come up with the ritual on the spot without consulting with the others. The ritual must invoke a specific sceptical reaction in other participants. Namely, their reaction must be: "That will work?" and not "That will work!" nor "That's brilliant!"
- If all other participants think that "That will work", the ritual will fail
- Magic is a ritual that can be performed by anyone - nobody is special, anyone can do it
- The one who performs the ritual decides the effects of the ritual. The details of the ritual could be anything, varying from dancing naked to counting stars or riding a cow upside down..
- The effects of the ritual is limited by what could happen in nature however improbable it is. One could call down a meteor or create tsunami, recover from cancer.. but not make water flow upward or create a black hole out of nothing
So, the main obstacle is the unpredictability. One could do some stupid things to call the rain the first few times, but then it get repetitive and lost the scepticism. Lost the ritual, and get a backlash (fatal).
How do I weaponize this magic reliably? If my army (say medieval) went against another and I dig a few holes to hide the "magicians" to do their magic. How to get them to keep spamming spells without losing their scepticism and get a backlash that kill both them and my army?
edit 1 To clarify how this magic affect the world:
- If you called down a meteor, the meteor won't just appear out of nothing right after you finish the ritual. The meteor is actually already coming, your ritual is just coincidentally happening at the right time. So the meteor won't just drop on your enemies, because it takes time for a meteor come from 'not clearly visible in the sky' to 'suddenly on their head'. It has to travel from somewhere to earth.
- If you called a tsunami, the same thing applies. The tectonic movements or some geographic activities are already there. There will be an earthwake or something in the ocean and then a tsunami will form, then it will come for both you and your enemies of course.
- You called a storm of frogs, a hurricane somewhere fished some frogs and dumped it on your face at the right time.
That's why I said the effect must be possible in nature.