Keen edge

Keen edge is a transmutation spell that doubles an edged weapon's threat range.

Effect
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.