Vitriolic Spell (3.5e Feat)
From D&D Wiki
Vitriolic Spell [Metamagic]
The acid conjured by your spells are so corrosive that it damages creatures that normally have resistance or immunity to acid.
Benefit: The acid of a vitriolic spell is so potent that it ignores the resistance to acid of creatures affected by the spell. Affected creatures with immunity to acid attacks still take half damage. This feat can be applied only to spells with the acid descriptor. Creatures with the air subtype take double damage from a vitriolic spell. Creatures affected by a vitriolic spell are still entitled to whatever saving throw the spell normally allows. A vitriolic spell uses up a spell slot one level higher than the spell's actual level.