Lesser Invocation; 3rd; Eldritch Essence
This eldritch essence invocation allows you to change your eldritch blast into a brimstone blast. A brimstone blast deals fire damage. Any creature struck by a brimstone blast must succeed on a Reflex save or catch on fire, taking 2d6
points of fire damage per round until it takes a full-round action to extinguish the flames or the duration expires. The fire damage persists for 1 round per five class levels you have. For example, a 15th-level warlock deals 2d6 points of fire damage for 3 rounds after the initial brimstone blast attack. A creature burning in this way never takes more than 2d6 points of fire damage in a round, even if it has been hit by more than one brimstone blast.
Source: Complete Arcane