Designing a contractual magic system?

[David Johnston2]

There are different kinds of contract

1. You want something done. You want someone dead, or healed, you want your vermin infestation evicted, you want your forest fire put out, you want your enemy spied upon, you want to find a deposit of gold or an aquifer, you want to be transported to another country in a hurry or to have the clock turned back so you can make a different choice, so you get in touch with something that can do it and make it worth their while. It might want blood, or a musical performance, a cow, a gourmet meal, a memory, a box of aromatic powder, a four leaf clover, the heart of a ferocious beast, or a promise of your firstborn child. For each task a new deal with a new reaction roll.

2. You want power. You want to be given one or more advantages, and in return you need to trade something that will probably be bigger, like a body part, your ability to sing and speak, a vow of servitude, all of your memories, your post mortem soul, your youth, your beauty, your sense of humour. Usually you take on a disadvantage in exchange for your advantage.