Source: Dragon #313
Your ability to agitate acidic substances results in especially caustic versions of spells and powers that have the acid descriptor.
Prerequisite: Any two spells with the acid descriptor, matter agitation.
Benefit: This feat can be used to enhance any spell or power with the acid descriptor. In addition to dealing its normal damage, an effect enhanced by Agitated Causticity literally softens up its target. This effect does not represent damage to armor, shields, or body parts that provide a natural armor bonus; it is a function of the acid's effect on the target creature itself.
For each 5 points of acid damage the target takes from such a spell or power, it must make a successful Fortitude save or take a -1 cumulative penalty to its Armor Class. The DC for this save equals the save DC for the spell or power (or the save DC it would otherwise have if no save is allowed). All other modifiers to AC apply normally.
The penalty from this effect decreases naturally at the rate of 1 point per day. Any effect that heals ability damage may also be used to reduce or eliminate the penalty by the same amount.
To apply Agitated Causticity to a spell, the caster must expend 3 power points per spell level. To apply it to a psionic power, the manifester must spend 1 spell level per power level.