Geasing single spells, good idea? bad idea? Options

[Jason Farlander]

I was just thinking about the fact that adepts can voluntarily accept geasa on specific powers for a reduction in cost. Mages, however, do not have this option - geasa are only used to offset magic loss. It struck me that, given that mages have much finer control in their ability to design spells than adepts have in designing powers, it would actually make a great deal of sense if mages were able to incorporate geas-like limitations in the design of a spell, and that such limitations would make those spells easier to learn and/or cast.

Having decided that, on an abstract, intellectual level, it should be reasonable for this to occur, I'm trying to figure out if and how this would be introduced in a way that remains balanced. Since I dont like the idea of using the adept's cost * 0.75 formula - it seems that this would be potentially overpowering - I was thinking about making it similar to the existing fetish/exclusive limitations. It seems to me that geasa are more limiting, overall, than the fetish limitation (its easier to replace a fetish than it is to replace the closest geas approximation - a talisman). At the same time, it seems to me that geasa are less limiting than the exclusive modifier. The fact that you could potentially combine both an exclusive limitation and a geas limitation makes me very, very hesitant to set it at a -2 modifier, and the convenience of being able to apply two limitations to a spell without making it exclusive justifies, to me, setting it at a -1, even though its more limiting than the existing -1 modifier. I do not think all three modifiers should be possible to apply to a single spell.

Here is what I tentatively propose:

The geas limitation can be applied to any spell in the same manner as a fetish limitation, and grants a -1 to either karma cost or effective force for calculating drain, as does a fetish limitation. This limitation can be combined with either the fetish limitation or the exclusive limitation (but not both); however, each limitation must apply to a different aspect of the spell (both limitations can not be used to reduce the same thing). If a both talisman geas and a fetish limitation are applied to a spell, the talisman must be a separate object from the fetish.

Thoughts?