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. 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. (Treat shuriken as arrows, rather than as thrown weapons, for the purpose of this spell.)