The original idea was that someone would get a hit on Billy to put him in some danger (it can be as bad an injury as we like, and even if it was an intended kill shot the magic could potentially make it non-fatal, so intent doesn't really matter), and then a second attempt would be blocked by Teddy. Teddy getting hurt protecting Billy would be enough to snap him out of it and he'd release the spell.
This can be tweaked as needed but the basic formula of at least two injuries happening is pretty much what we need.
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The original idea was that someone would get a hit on Billy to put him in some danger (it can be as bad an injury as we like, and even if it was an intended kill shot the magic could potentially make it non-fatal, so intent doesn't really matter), and then a second attempt would be blocked by Teddy. Teddy getting hurt protecting Billy would be enough to snap him out of it and he'd release the spell.
This can be tweaked as needed but the basic formula of at least two injuries happening is pretty much what we need.