Penetrating Weapon

Enchantment 
Cutting, impaling, or piercing weapons with this enchantment pass through armor as if it were butter. Specifically, the spell grants the weapon an armor divisor (p. B268). Such a weapon does not do extra damage to unarmored foes. Hardened DR defends against the armor divisor conferred by this spell as normal.

Energy cost to cast: Depends on the armor divisor conferred on the weapon.

Double the cost if the subject is a missile weapon (e.g., a bow). Divide the cost by 10 if subject is a missile (e.g., an arrow). If the item falls in more than one class (weapon, missile weapon, missile), use the higher enchantment cost. In addition to weapons, any sort of cutting tool may be enchanted. Note that this spell may be recast at a higher level as per  Accuracy. 
Prerequisites: Enchant (VH) and Find Weakness.