A Shadow Item is a special weapon that is made in cooperation between the Ivory Tower and the Black Anvil Guild. It is created by magically projecting the power of a weapon onto another catalytic object. While that means that it can only be used while the magic charge remains, it has made it possible to mass produce projectile weapons.
Its performance is no different than if you used the original weapon, except that it cannot possess the special ability to absorb souls or augment through a Life Stone. Also, although it acts as a weapon, it is actually in the magic category -- once opened, it cannot be passed on to someone else. Consider it an aid on your new path until you lay your hands on some real weapons.
If you have a Shadow Item Coupon, you can receive a Shadow Item right for you.