I have a question which is threading me for some weeks now.
We have some spells, - most examples which I'm aware are Rego Terram spells - with Rego technique which produce massive things, such as earthquakes.
I'm aware that a spell affecting you needs to breach your magic resistance. And if it's aimed it must do an aiming roll (and if the aimed thing is a magically created thing, it also needs to penetrate).
But then we have the earthquake.
Earthquake target is the earth, and it begin to shake. The description say things as "those who stands on the shaking ground musta make rolls to avoid (consequences)".
And there I'm: what if an earthquake, with no penetration occurs. A magus (not affected) is walking on the earth.
Due to the earth quaking, a pit open. It's a clear case of when the magus A should do a roll to avoid consequence (falling, being crushed by the earth...).
But since he is not affected by the spell, he just is not suffering the consequence, so, he is not falling, not crushed, all things which seem unrealistic since it mean that magic resistance would allow a magus to fly over a magically opened pit (by Rego)...
How do you resolve this situation?
I have been trying and my idea was that: the Rego is on the earth, but not the immediate earth on which is walking the magus, but more the earth under it, and that the earth on which he is walking (and may fall in) is earth which follows the natural movement of the magically affected earth.
A kind of analyse as by rego ignem spell: if you move a natural fire on a magus, he won't take damage since it's a magical movement, and the fire will fall on the ground. But then,
if your spell stop, the fire is natural and burns him.
if your spell has a duration longer, it can starts another natural fire (if for example the magus is walking on straw) and that fire can burn him.
If I apply it to ReTe:
if your spell stop, the consequences (pit opened, mudslides...) are natural,
if your spell has a duration (like most ReTe spells which I'm thinking of), your magical control moves, but what was started (natural quakes following the magical quake) are not.
Well, in the case of an earthquake, the ground under the magus who resisted would not actually shake, remember, your parma magica extends a lit further than you, but other than that...
Yes, if you open a pit magically, magic resistance does not actually prevent you from stopping into that pit, it only prevents the magic from opening the pit directly under you. Now, causing the edge of the pit to crumble under a magus that stands next to it would be an uncertain tactic, and would require interestingly high Aiming rolls at the very least; I would also allow the magus in question Qik/Dex + Athletics rolls to move back before falling.
But you are quite right about "side effects" your Rego Terram spells being able to target a magus without worrying about magic resistance. Not that I would allow natural quakes to follow magical quakes (usually anyhow, there are always special circumstances), that would be way too easy. An trying to intentionally use these natural effects would usually require fairly difficult Aiming rolls (i.e., -6 or worse penalties to actual hit whatever you aim at) - it is one thing to control the earth you affect with your magic accurately (ofc, even that may require Finesse rolls), it is quite another to use that control to cause secondary effects that you like where you like.
A Rego effect cannot make the target invade the Magus' close personal space. It can move the earth away from him (except for the dirt clinging to his shoes) so an earthquake spell will cause the Magus to sink a little with each oscillation - if he keeps moving it will be barely noticeable although that would require balance checks for the constantly changing, uneven ground but if he stays still he could end up in quite a deep hole - he can be buried by natural collapse of the hole but cannot be crushed until the spell ends. If a crack opens up underneath him he can certainly fall in and be hurt by the fall but see above about crushing.
Targeting difficulty depends on the area of the earthquake - if it many yards across then it is a trivial matter to make sure it includes the ground the Magus stands on - I would not ask for a targeting roll unless he is moving quickly. If the spell design includes targetable cracks than a targeting roll with no penalty is needed (there may be a bonus if the crack is very big) but if cracks are just a happenstance result of the earthquake then give the Magus a dodge roll each round and he falls in a crack if he botches that or any movement related roll.
Personally I'd allow the magus magic resistance to protect against rocks jumping up at him, but not to avoid falling into holes. These would be done with mundane athletics checks. Having 'partial resistance' (i.e. an athletics bonus) if trying to move through the space makes sense. The earthquake 'ripple' would just not ripple where the magus is - like a wave parting around an island.
I'd treat holes opened up the same as Pit of the Gaping Earth: if they close again as part of the spell the magus won't get crushed, but he will still fall just like anyone else. That said, most earthquakes don't create big holes except in films. They do cause collapsing buildings, and those would definitely not be resisted!
Rego-moved fire still burns just like any other fire. The damage isn't from the magical 'impulse' on the fire like it is on a magically moved rock, its from a naturally occurring property of fire that the magic isn't modifying.
That said, I wouldn't allow a ReIg spell to sustain a fire in any round in which it has no fuel. So if your sustained ReIg spell involved it leaping from torch to torch, sure. If you want a ball of hovering fire that you can wave around people, I'd rule it goes out after the first 'swing' because Rego doesn't allow a state that is unnatural, and its definitely unnatural to have a fire without fuel that lasts for more than a moment.
Earthquake shakes the earth under the magus, as the Parma only protects against magically affected or effected things effecting the magus personally. However, even if the earth under the magus is not moving directly, it is moved indirectly by the earth next to it (if that makes sense).
force ----> earth>earth>earth>earth>"protected" earth<earth<earth<earth<earth <------force
magically moved rock------------>bang(hits)mundane rock------------> magus's face
In Canon ArM5, this is common. For example, the Re and MuTe effects that shatter stone allow for rock fragments displaced as a secondary effect of the first motion to bypass magic resistence (check the Terram section of ArM5 for examples.)
There are rules for aiming very large effects. You get a +6 to your aiming roll for every size increase above the basic individual (and the ST has discretion to modify it further as needed; the Auram base individual is big enough to merit an increase all of its own, while the base individual for a diamond is quite small).
So I would say that if you are trying to affect someone with an earthquake, you do have to make an aiming roll. Crest of the Earth Wave, for instance, has a +2 size modifier, giving you a +12 to your aiming roll. Earth Shock has a +4 size increase, that's a +24. It may seems like an automatic hit, but there's always the chance you'll botch the aiming roll and ... well, missing the target is the least of your worries at this point.