Greater Invocation; 6th; Eldritch Essence
This eldritch essence invocation allows you to change your eldritch blast into a vitriolic blast. A vitriolic blast deals acid damage, and it is formed from conjured acid, making it different from other eldritch essences because it ignores spell resistance. Creatures struck by a vitriolic blast automatically take an extra 2d6 points of acid damage on following rounds. This acid damage persists for 1 round per five class levels you have. For example, a 15th-level warlock deals 2d6 points of acid damage per round for 3 rounds after the initial vitriolic blast attack.
Source: Complete Arcane