Fairy Law

Fairy Law (妖精の法律, フェアリーロウ, Fearī Rō) is a Spell that is distinct to the Fairy Tail Guild. It is one of the three legendary Fairy Magics.

Description
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 and is meant to discourage further action from enemies, and is not something that should be released whenever the Caster feels like it.

But like all great magic spells, it comes with equally risky effects if performed incorrectly or in its incomplete state (which requires 10 years of training to complete said spell). The most notable example of this would be Mavis Vermilion. In her attempt to save her friend Yury Dreyar from the evil influence of the Tenrou Jade, she used an incomplete version of it that was simply called: Law whom she learned from Zeref during her training. This unfinished version not only caused Mavis to collapse shortly after casting it but even stopped her growth entirely, forcing her into permanent youth.