This spell makes a weapon magically keen, improving its ability to deal telling blows. This transmutation doubles the threat range of the weapon. A normal threat range becomes 19–20. A threat range of 19–20 becomes 17–20. A threat range of 18–20 becomes 15–20. The spell can be cast only on piercing or slashing weapons (and it does not stack with itself). If cast on arrows or crossbow bolts, the keen edge on a particular projectile ends after one use, whether or not the missile strikes its intended target.