During city creation, my players came up with the idea of having a particular area be a magical null zone, a la the aquarium during the Denariians' Archive kidnapping attempt
; part safe zone (there's not as much power available here), part prison. I thought I'd ask around and see if folks here had any good ideas for how to run a location like that.
1) External magic sources are cut off; you have what you came in with. How would this operate in play? A limit to how many shifts of power worth of spells you can cast? And if so, how would one set a reasonable default limit, and a reasonable "You can pull in a little bit of extra power, but you'll be casting dangerously until you bleed the excess off" effect? For the second, I'm inclined towards a variation on "you can pull in up to your conviction extra shifts of power, or roll to draw in power and control it a la thaumaturgy but without the spell; all spells that you cast have an added difficulty of the extra shifts of power you have stored over your normal threshold"; it makes holding more than a few extra shifts of power really, really dangerous, but it's possible if you're desperate. What I don't have is any idea what a reasonable base threshold ought to be.
2) Some powers probably shouldn't work as well there... but which ones? Sponsored Magic being right out unless you prepared the spell in advance would make sense. How about faerie glamours that create things out of ectoplasm pulled from the Nevernever? Or other shapeshifting, ditto? Which magics are inherent (and therefore preserved), which are penalized, and which ought to just be out of the question?
3) How do I create a feeling of "Magic is hard here?" For an actual mage, limiting the amount of power they can draw or making all spells more expensive (for more of a low-level anti-magic-field than an actual hard cut-off) seems pretty reasonable. But for powers that don't normally have a roll (like shapeshifting), what do I do? I can add an aspect to the location easily, but that will only help people trying to take advantage of it; it won't make life harder for everyone else. And... should it? Or is this really something that ought to just be color?