Baneful Blast

Lesser Invocation; 3rd; Eldritch Essence
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.

Source: Complete Mage