When activated, a bright light envelops the area and inflicts heavy damage on whoever the caster perceives from their heart as an enemy, leaving friends and bystanders completely unharmed. The choice between friend or foe is decided by the user's heart, and they cannot lie to this Magic. It is considered a very powerful Magic and is one of the rare legendary spells, and also one of the three great Fairy Magics.
The spell's strength was so great that it defeated Jose Porla, a Wizard Saint, in a single blow. According to the second master of Fairy Tail, Precht, Fairy Law is a deterrent Magic, meant to discourage further action from enemies, and is not something that should be used lightly.
Setting Law as its basis, Mavis modified and improved it so that users wouldn't suffer from its side effect.