The Earth's Carbuncle (Page 156) species a target of "Part" - it causes a 1 pace area of ground to explode underneath a given person.
How would one modify that so that the ground under multiple, separated persons could be targeted to explode.
Essentially I'm wondering if you can just change the target to "Group" (the group being 1 pace sections of ground under a set of people) or whether there is some more elaborate calculation that needs to occur.
On a related note - how is the damage for that spell or "The Crystal Dart" determined?
I would increase the magnitude by +1 to increase it by size... so instead of one part, you would affect 10 parts, for a ReTe20 effect.
You're still just targeting the "part," rather than a group... you're just targeting a larger number of parts. You're not targeting a group of stones, or a group of individuals-- you want many discrete portions of ground...many parts translates to a part target with a +1 size modifier.
I think the "Part" in all Rego Terram spells refers to "Part of the Earth as a Whole." The maximum size of the area of effect is still determined, I believe, by the guidelines on pg 152, eg 10 paces of soft earth, 1 pace of stone. Every magnitude would multiply this amount by 10.
I would say that choosing who get targeted in that area would fall under the aegis of "fancy effect" and add a magnitude, and maybe a targeting roll.
I think the damages are determined by "what sounds right for the effect." The level of Creo Ignum to do +15 damage is 10, aparently the level of Terram to do +10 (Muto and Rego) is 3....which seems a bit low, but there it is.
I would suggest +5 damage per magnitude you add to the base level of the spell, that is not used for any other effect.