You give a weapon the undead bane special ability in addition to any other properties it has. Against undead, your weapon's enhancement bonus (if any) is +2 higher than normal, and it deals an additional 2d6 points of damage. The power has no effect if manifested upon a weapon that already has the undead bane quality.
Alternatively, you can affect up to 50 arrows, bolts, or bullets. The projectiles must be of the same kind and they have to be together, such as in the same quiver. A projectile (but not a thrown weapon) loses the special ability after it is used in an attack.
The weapon is considered good-aligned for the purpose of overcoming damage reduction.