As a reaction, which you use when you are hit by an attack or missed by 4 or less, your skin erupts in a swollen, pus-filled bump that bursts open, shooting acid. If the creature who damaged you is within 10 meters, it must make a Fortitude saving throw.
On a failure, it takes 2d8 acid damage immediately and additional acid damage equal to half the amount of acid damage dealt at the end of its next turn and its armor or natural armor is corroded 1 until the end of your next turn.
On a critical failure, it takes twice as much damage and its armor or natural armor is corroded 2 until the end of your next turn.
On a success, it takes half as much damage.
On a critical success, it takes no damage.
You can increase the damage by 2d8 for each additional mana expended.