One of my players is considering something like this (specifically, her inspiration is Yuna of FFX), and we decided sponsored magic, with the sponsor being either the summoned creatures in aggregate or their ruler (i.e., a Bahamut analog) and smooth over the whole thing about beating the creature's will as part of the sponsor benefits--the deals have already been made, the summoner is just calling in a quick favor.

What I came up with for the tentative Evothaum (character's still in the hypothetical stages), amounts to:
Base, 1-shift spell gets you a minion for one scene, with no refresh spent, and a skillcap of 3. So something that might be useful as a second set of hands, but not really powerful.

You can raise the creature's skillcap by 1 for each additional shift, up to 5 normally (I'm thinking some kind of diminishing returns to discourage going above that--maybe you need an additional 2 shifts to bump it to 6, an additional 3 to get it to 7...or maybe they can't summon something with a skillcap above their Lore?).

Additional shifts can be devoted to powers, with one refresh equal to one shift.

So, on this model, her "Shiva" summon would have a 5-shift cost--skillcap up to 5 for Weapons, with an ice-based breath weapon. Since the summons are ectoplasmic bodies that only last for a scene, the Recovery powers can be cut out entirely to save costs. The Catch is factored into toughness powers (the ice summon would be obviously weak to fire, etc.).

She's considering having each summon as a Rote spell, with attendant focus items to help cover the power costs.

Thoughts?

They're trapped. 

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Remember Ghost Story, specifically the cemetery!