You create a small sphere of acid that you fling at a nearby target. It will continue to do damage over time and lower base defense.
The acid hits the target for 1d10 damage which continues to burn for half the damage for 2 rounds. Furthermore, the acid will corrode the defense of the target's defensive bonus by 5 while the acid is effective.
If you successfully hit the target with an acid sphere while the target is still under effect, then the damage for the next spell will be doubled - meaning the burn for the 2 rounds will be the full damage instead of only half.
The effect can only happen once to a target per combat.