Random Magic Weapons (any level)

This enchantment doubles the threat range of a weapon. For instance, if it is placed on a longsword (which has a normal threat range of 19-20), the keen longsword scores a threat on a 17-20. Only piercing or slashing weapons can be enchanted to be keen.
Aura: Moderate transmutation
Caster Level: 10th
Requirements: Craft Magic Arms and Armor, keen edge
Price: +1 bonus
Source: Dungeon Master's Guide v.3.5