Keen Edge

This spell makes a weapon magically keen, improving its ability to deal telling blows. This transmutation doubles the threat range of the weapon. A threat range of 20 becomes 19–20, a threat range of 19–20 becomes 17–20, and a threat range of 18–20 becomes 15–20. The spell can be cast only on piercing or slashing weapons. Multiple effects that increase a weapon's threat range (such as the keen special weapon property and the Improved Critical feat) don't stack. You can't cast this spell on a natural weapon, such as a claw.