Fairy Law (妖精の法律, フェアリーロウ, Fearī Rō) is one of the "Three Great Fairy Magics" distinct to the Fairy Tail Guild, and an improved version of Law.
When activated, a bright light envelops the area and inflicts massive damage on whoever the caster perceives from their heart as an enemy, leaving friends and bystanders completely unharmed.
Fairy Law is one of the most powerfull spells in the show. We know for certain that both Makarov and Laxus can use this spell. And the one Makarov used was so powerful that he one shot a wizard saint!
So why do you think nobody used it during a war. Even in a full out battle between allies and foes you can still shoot it to the entire battlefield cause it wouldn't hurt any of your friends. It most likely wouldn't one shot any Spriggan but it would still take out a lot of fodders, thus making the battle much easier.
Is it a plot reason? Hiro made the spell overpowered so he doesn't want to ruin the story? Or there is some deeper reason behind it? I read online that maybe some Spriggans have the ability to influence mind and thus making Makarov and Laxus to think that they friends are actually their enemies and FL would hurt them too.
So what do you think? Plot or some deeper reason?