One of my players, specialized in Terram, wants to invent a spell in which he makes a throwing movement with his arm and a small pebble comes forth and begins to grow to the size of a catapult projectile at someone or something.
Now, I told him it would be a CrTe effect with a requisite on Rego, so I was thinking somethine like this:
Base 3, +2 Voice, +1 for the Rego requisite = 10
He argued however that Ball of Abysmal Flame had a similar effect (create a ball of the appropriate Form that is thrown at a target) and doesn't require the Rego requisite. Well I told him that the "throwing" of the Abysmal Flame was merely aesthetic and he was convinced.
But I'm still unhappy with the end result, and I have no idea on how to calculate damage on that one. At first +10 seemed fair, judging by the Wielding the Invisible Sling spell's damage, but with a level 10 spell, it seems just too much!
A possible question is whether a stone does damage according to matter or momentum? In a modern day scientific context it shouldn't be too hard, but how will you approach it in a Mythic Europe one? The rules arent entirely clear. Ignem guidelines have a clear take on magnitude and damage but many of the other guidelines do not. The obstacle chapter has rules on falling which arent clear about matter or momentum either - and in any case falling toward the ground is different than having somthing hurled against you (in the sense of how to percieve it in terms of metaphysical cause and effect) in terms of acceleration or the contrary.
I know, I know - not any answer but just more questions...
I think in the medieval paradigm momentum was mostly irrelevant, and the damage would be calculated according to the size of the stone (a base individual of a single cubic pace), but I may be mistaken, and in fact, the table for missile weapons suggests otherwise, as a sling deals +4 damage, while a thrown stone deals a mere +2...
I thin comparing the principle to 'ball of abyssal flame' is a bit of a non starter because fire hurts - stone doesnt. The act of launching the fire is pure cosmetic in that particular instance.
Now, in your case, creating 'at' someone desont hurt; at leats not without a targetting roll because being in contact with stone isnt necessarily gonna hurt.
Even in the medieval sense it is the force with which the stone is thrown that causes the damage; the other factorbeing the size of the stone. We have the biblical story of David & Goliath that proves to us force is a factor too becaues david has a sling for his small stone.
I woudl argue thatin your case the size of the stone and its apparant 'growth' is the cosmetic bit. I would instead suggest you go for an enhanced version of the invisible sling, simply adding a creo requisite becaues you are using a magically created stone. Base size fo creo terram(stone) is 1 cubic pace, so you got no problems there, and the end result is lvl 15. As you are creating the item too you need not target the spell either; in fact that would be pointless as it is magical it would need to penetrate anyway.
Well, I see no problem with the spell as presented.
Wielding the invisible sling is ReTe, and throws any nearby object that could normally be thrown by a person (which isn't really an hindrance, you could even carry some slingstones with you, or little metal balls...) with a lvl 10 effect.
Your spell does the exact same damage. Of course, if doesn't rely on a nearby object, but it uses 2 techniques, which makes the spell less effective in the long run.
Yeah there is a long list of spells which offer means of circumventing parma by 'launching' missiles and targeting things instead of making the hitting happen by magic.
I must admit I am not very keen on these spells as they seem to 'fudging' the principle personally and letting 'science do the damage' - gravity being a force not to be discovered for a few hundred years after our cannonical setting.