So, the gamebook says that it's unlikely to use an outside source of power for Evocation, but it might be possible (and we have a canon example in Harry using lightning to zap frogman in Storm Front). So I'm curious as to what ways this might be implemented.
The obvious way, of course, is to Invoke the scene aspect representing the source of power for a +2 to the power of the spell.
One idea I had, though, is to let the player invoke for effect, and give them a set power rating that they don't have to spend a mental stress for--but they don't get to choose how much of that power they want to use. So if you invoke the power of a Thunderstorm, you might get a free Weapon:8 spell, but you have to roll Discipline to control all 8 shifts, and deal with either the fallout or backlash as it comes.
Any other interesting ways to deal with this that you guys have found?