Leg crusher: A disabling spell that opens a sinkhole below an opponent and quickly snaps back, crushing leg bones.
Feasible? I'm worried that the DM might accidentally make me kill someone by opening the hole too big and crushing a torso or something like that. Is this something I should be wary of?
Yes and no. Definitely feasible in the way you describe it, just depends on what exactly you want to do with it. Basically, you have 3 options with this type of spell: attack, maneuver and block.
First, breaking legs is a consequence, that would result from an attack. If the attack isn't strong enough though, that won't happen, the target will just take some stress (which will still wear them down over time, but it isn't leg breaking). A broken leg probably starts to make sense at moderate consequence levels, which will take some doing to get to in one shot.
I would probably call the spell
Sinkhole and make it either a maneuver or a block. A maneuver, if you want to use the aspect on something else afterwards, a block if you want to keep the target from moving. You could also go for a "tag for effect" with the maneuver, to force a compel on the target (I'll explain more later, if you like).
Coinshot: This is stolen from Mistborn. Harry uses his sword cane as a focus for spells involving magnetism. Are there special rules governing this? Do magnetic spells always attract metal towards the caster, or can I be more versatile with it? For example, can I take a handful of ball bearings and shoot them outward towards a hostile?
If not, magnetism still has the advantage of disarming opponents and maybe I can even hit people with metal objects when my opponent is between me and the object itself.
Magnetism can do a lot of crazy things. I once created a spell that had the wizard use magnetism to melt a metal pole with induction heat.
It all comes down to how your wizard understands what he does. Does he have a basic understanding of magnetism (it can push/pull metal) or does he know more? Shooting ball bearings is a great way to use it. This sounds like a very good standard attack spell.
Dampening Feild: Can I use water to dampen the magic of others? Say, pull ground water out of the earth and interpose it between me and an oncoming evocation? Can my earth magic help me pressurize the ground water to assist pulling it out of the soil?
This would be a straight up block against magic. Having the earth magic help is certainly possible on the narrative side, but you can only use one set of specialization or focus item bonuses for a spell. And since you are using water to dampen the magic, I would judge this to be a water spell.
All of these ideas are pretty high-octane. Would they cause me an unreasonable amount of stress?
They will cause you as much stress as you make them, though 1 stress minimum. Maneuver spells have to be power 3 for a fragile aspect (which goes away after being used) or 4 shifts of power to make it last. Attacks and blocks can be as high or low as you like. You take 1 shift of casting stress for each spell, plus 1 shift for each shift of power beyond your conviction.
So if you have a conviction of 3 and you want to cast a 5 shift block, you take 3 shifts of casting stress. If you have a focus item or specialization for power that is relevant to that spell, it is added to your conviction for calculating the stress. So a focus item for +1 water power on the above spell would reduce the casting stress to 2.
Hope that wasn't too much all at once. If I can clarify something, I'll be happy to do so.
