All critical rolls against evil foes are automatically successful (so every threat is a critical hit). The weapon negates the damage reduction of evil creatures and is capable of striking evil incorporeal creatures as if it had a +1 enhancement bonus. Also, the weapon is considered blessed, which means it has special effects on certain creatures. Individual arrows or bolts can be transmuted, but affected projectile weapons (such as bows) don't confer the benefit to the projectiles they shoot.
This transmutation can't affect any weapon that already has a magical effect related to critical hits, such as a keen weapon or a vorpal sword.