Nice idea
I'm happy to contribute, how's this?
Name: 'What did I eat?'
Type: Water (biomancy) evocation, offensive manoeuvre
Power: 4 Shifts of power, 3 for the aspect, 1 to make it sticky.
Duration: Once scene as the aspect is sticky. One free tag.
Opposed by: Targets Discipline (if the targets Discipline is above +3 (Good) then adjust the power of the spell accordingly).
Effect: The target's stomach suddenly feels particularly treacherous and the target now desperately needs the loo.
Variations: +2 shifts to make it zone wide?
It's quite possible that only a PC with some sort of medical knowledge may be able to cast this spell or, at least, a reason why they'd understand the human body enough in the required way. This is a particularly subtle spell and might well be useful in a social conflict too.
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Evilnerf: Shouldn't magnetic block require +2 for allies instead of just +1?
Also, to reuse your magnetic block to cast steel rain, you need to show that the spell hasn't been used. I'd say that even if it was used in the first exchange, it'd be fine to reuse in the second as long as it hasn't blocked anything however you should lose 1 shift of power for the duration that's been spent. I can't see any examples that show if this is possible though. Anyone?