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If I cast fireball against a "wasps nest" (say 40 low power mooks)

When making effect tests from willforce in an instance where I am hitting multiple targets like the 40 above is that 40 strain or 1?
karma of course is the other matter is karma fixed or variable when hitting multiple things with one action. Lightning bolt would be another example. For extra successes or extra threads for multiple targets. how much strain, how much karma when casting lightning against multi-targets

This is my interpretation.

The strain and the Willforce step replacement happens for each "Effect test". You only roll once for Fireball. In contrast, you roll multiple times for Stone Rain - once per round. In this case, you would need to decide to trigger Willforce for each roll.

Bone shatter seems to best match that "multiple Effect tests for a group" description, as with that spell you target a group with a Spellcasting roll (vs highest MD), but you then get 1 Effect test per Success scored (no more than 2 Effect tests per target). Not many spells work like this, though.

one effect test is also to speed the game up as well if you only roll 1 effect test on acid raid. that his an army of 100 solders that doesn't mean your now going to roll 100 effect tests to add willforce in. Its still only 1 effect test

chris happens to be my gm when this discussion came up. lol

It was discussed in Ed3, and as I see was changed only little in Ed4. As Telarus already wrote, it is meant to work with Effect Tests. So in case of Fireball 1 Strain and 1 Karma will make all targets suffer damage from EffectTest (its also true for all spells where 1 effect test is used for all targets).

In case of Bone Shatter and spells that generate separate Effect Tests you need to use Willforce on each test for best results.