This weapon is enveloped in a shroud of darkness.

Whenever you score a critical hit with this weapon, a wave of blackness washes over the target, causing it to become shaken (no saving throw) for 5 rounds. This effect activates even if the creature struck is not normally subject to extra damage from critical hits. This effect doesn’t stack with itself or with any other fear effects (it can’t render an already shaken creature frightened, for example).

Prerequisites: Craft Magic Arms and Armor, fear.
Cost to Create: 4,000 gp (plus 300 gp for base item), 320 XP, 8 days.