D20:Keen Weapon Enhancement

Keen: This ability doubles the threat range of a weapon. Only piercing or slashing weapons can be keen. (If you roll this property randomly for an inappropriate weapon, reroll.) This benefit doesn?t stack with any other effect that expands the threat range of a weapon (such as the keen edge spell or the Improved Critical feat).
Moderate transmutation; CL 10th; Craft Magical Arms and Armor, keen edge; Price +1 bonus.

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System Reference Document -> Magic Items: Weapons