In the book Storm Front Harry Dresden channels the power of a storm to help destroy a demon.
One of the PCs in my game wants to do something like that but I can't figure out how to mechanically make it work, has anyone worked out a way for the mechanics to allow for it?
I know in book he's not using Thaumaturgy to accomplish it, so it must be Evocation. So for this, my question would be:
How do you model a Wizard PC channeling the power of a storm, or any external power sources for evocation?
Yep, it's evocation. Harry doesn't have the time for any ritual, and is just grabbing everything he can to throw into a blast.
Channeling the lightning is just tagging the scene Aspect Lightning Storm. But that's just grabbing the power, which is a bad idea and nowhere near enough to actually make use of it in an evocation. Harry is also throwing lots of Fate points around at other Aspects to actually pull it off using normal evocation rules and not just make himself a suicidal lightning rod. To quote Rick Neal's DFRPG Q&A 10:
That was Harry dumping Fate Points to tag a whole bunch of Aspects, including the Lightning Storm Aspect that was on the scene. And he still wound up taking a whole bunch of backlash!
Recall that by that point in the story, Harry had been bruised, beaten, generally knocked around, and been the loser in many conflicts. It was also at the peak of a story arc, meaning he and the game had accumulated a number of meaningful Aspects that were all coming into focus in this one moment (which is why it was climactic in the book). Doing something so stupidly dangerous also very likely counts for at least one, if not more, player-initiated compels, which are worth Fate point. That doesn't even count all the compels the player might have asked for or had imposed earlier, building up Harry's reserve of plot power.
Harry was flush with Fate points from accepting all those compels and surrounded by all kinds of Aspects representing his state of mind and the story situation. As the natural climax of an arc, we can safely assume that there were so many relevant Aspects and he had so many Fate points, that he could just blow them all in one big boom.
For this example, I will use a wizard character with 5 Conviction and 5 Discipline.
The way casting an evocation spell works, is you cast the spell with a number of shifts up to your conviction (5), then try to control (and in the case of attacking something, aim) it with your Discipline (5). Assuming you're successful in controlling all the power, you take a 1 shift mental stress hit. You can call on more power than your conviction would allow, but for every shift above, you take an additional stress in backlash. Not only that, but every shift of power you can't control with your Discipline roll, if you want those shifts to stay in your spell, you have to take another shift of stress in backlash. On average, attempting to cast a 7 shift spell, would result in a 5 stress hit to yourself.
What I think calling on a storm would do, is it would give you a massive boost to your power (Conviction), but not your ability to control it (Discipline).
For a working example, our theoretical wizard needs to blast a toad demon into oblivion, right then and there, but also knows that the base average damage (10 shift hit - 5 power from conviction, and 5 to hit from Discipline) isn't going to do it. He calls call on the power of the storm.
The power drawn in gets a boost of 9 shifts, but it's an all or nothing thing. He can choose how much power he brings to the table, but if he wants to use any of the power of the storm, he has to use all of it. So, the cast would be done with a minimum of 9 shifts, and a maximum of 14. For that spell, in game terms it would work the same way as getting a 9 shift boost to conviction skill. The wizard would receive no backlash from calling more power than he's used to, but he'd most likely get a ton from trying to control it all.
Casting the spell- a lightning evocation attack- the wizard draws in 3 shifts of power on his own, with the added 9 from the storm, for a total of 12 shifts that need to be controlled. As stated, Discipline is 5 and the roll is all blanks (0). So the wizard takes 7 stress in backlash from the uncontrolled power, and manages to pull off a weapons:12 attack with 5 to hit.
