Foul is an action unlocked by questing at level 70. It's available for Black Mage.“
Deals unaspected damage to a target and all enemies nearby it with a potency of 650 for the first enemy, 10% less for the second, 20% less for the third, 30% less for the fourth, 40% less for the fifth, and 50% less for all remaining enemies.
Can only be executed while under the effect of Polyglot. Polyglot effect ends upon use.
— In-game description