Killing with Magic or Otherwise: You opposition is going to be mostly mortals for my adventure which means there will be a risk of Law Breaker coming up. While you do have narrative control over your attacks there are times when this seems unreasonable. Every enemy will have a secret Stress Threshold which, if an attack brings them over that level they will die, this will only, however, apply to attacks of Weapon: 3 or greater.
The problem with a rule like this is that it really doesn't make a lot of sense. A broadsword is a weapon 2...but somehow something that can slice someone in two or pierce their heart can't kill them? Yet if you grab a two-hander then boom, they could be dead? There's a big level of inconsistency here.
I've sort of been advocating the narrativist side very heavily in this thread, since that seems the more right of it (and at first other people weren't doing it). I CAN see how sometimes it doesn't make a lot of sense to declare the attack didn't kill. Shooting people (weapon 2 with pistols) seems like a somewhat bizarre way to not kill them but still take them out. Once or twice doing that...not bad. Going into a building full of people and everyone shooting them with guns and no one dying...that could get so bizarre it is hard to take seriously, especially if it happened time and time again. So that can hurt the game.
That said, it IS up to the player, technically, and I think it is important to maintain player input here. Probably the best way would be with aspects. "This is a Lethal Attack" is something a GM can compel and a player could only get out of it by spending a fate point makes more sense if the player is going around making a lot of attacks with lethal weapons and saying they never kill anyone. That said, seems like magic is generally more flexible and some melee weapons are reasonably enough non-lethal (broad sword and the flat of the blade*). It does seem like something that needs to be discussed a bit, but in all cases a GM shouldn't spring lawbreaker on someone. I've been gaming for over 15 years and I know well the GM and a given player don't always see eye-to-eye. It's important to be clear about big things like this before they happen so you don't end up with a big fight (which sucks).
Hmm, I am still not convinced Invoking for Effect is a great solution. Taking people out (e.g. getting them to run or surrender) doing that is essentially using Invoke For Effect to produce a compel on multiple people if more than one is about...and that seems to be way too powerful for me. I like more using an zone-wide attack and describing it as an explosion centered on a bad guy...the shifts people away from that take are concussive force, noise, and so forth (won't kill anyone, but will scare the jujubes out of them causing them to get taken out). A properly narrated attack just seems like the better way to go here (though it poses still an interesting question for enchanted items...must they be created with lethality or non-lethality in mind?). If a player is putting forth effort to not kill people...then that should be what matters.
*Yeah, I know clubbing someone in the head isn't nearly as friendly as TV makes it out to be...nonetheless.