This weapon acts as though it has the spell detect evil upon it.
Book of Exalted Deeds
James Wyatt, Christopher Perkins, Darrin Drader
This enchantment is always active unless the wielder commands the weapon to deactivate (a free action). The wielder receives a +2 bonus on attack and damage rolls against any creature with a strong or overwhelming aura of evil. Bows, crossbows, and slings so crafted confer the righteous power upon their ammunition.