Brimstone Blast

Warlock Invocation
Level: Lesser, 3rd lv.
Components: V, S
Casting Time: 1 standard action
Range: 60 feet
Duration: 1round/5class levels
Saving Throw: None
Spell Resistance: Yes

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.

(Complete Arcane, Page 134)