Baneful Blast

Warlock Invocation, Eldritch
Level: Least, 3rd lv.
Components: V, S
Casting Time: 1 standard action
Range: 60 feet
Duration: Permanent
Saving Throw: No
Spell Resistance: Yes

You transform your eldritch blast into a baneful blast. This blast deals an extra 2d6 points of damage against creatures of a specific type. You must select the creature type when you first gain this invocation, and you can't later change your mind (though you can take this invocation more than once, choosing a new creature type each time).
You choose your creature type from the categories offered for a ranger's favored enemy, including a subtype if you choose humanoid or outsider.

(Complete Arcane, Page 134)