A Power key is much diffrwent then a spell key. They're much rarer, as they're granted directly from a godly power to his or her servants, the priests. They can change at the power's whim, and don't usually last longer than a week or so. They're also much more potent: Power keys cause the spell they affect to have maximum possible effect. Basically if there's a random factor in the spell (damage, range etc.) then it always is as large as possible.