Keen Edge

You make one metal weapon or up to twenty pieces of metal ammunition magically keen, improving its ability to deal telling blows. This spell increases the Critical Hit Range of a weapon by 1 for the duration, and can only be cast on a weapon that deals Piercing or Slashing damage. If cast on Arrows or Crossbow Bolts, the spell on a particular projectile ends after one use, whether the missile strikes its intended target or not.   At Higher Levels. When you cast this spell using a higher level spell slot, increase the Critical Hit Range by 1 for every two slot levels above its base level. For example, a Paladin casting this spell at 5th level could cause his Longsword to score a Critical Hit on a roll of 18-20, having increased the range by 2.
 

Transmutation Level: Runescarred Berserker 2, Sorcerer 3, Wizard 3, Duskblade 3, Nentyar Hunter 3, Wu Jen 3 (Metal), Sha'ir 3, Urban Druid 3, Metal 3, Components: V, S, Casting Time: 1 standard action Range: Close (25 ft. + 5 ft./2 levels) Target: One weapon or fifty projectiles, all of which must be in contact with each other at the time of casting Duration: 10 min./level Saving Throw: Will negates (harmless, object) Spell Resistance: Yes (harmless, object)   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).   Multiple effects that increase a weapon's threat range (such as the keen edge spell and the Improved Critical feat) don't stack. You can't cast this spell on a natural weapon, such as a claw.