The Broken shield is one of the two halves of the restored Shield of Arrav.
In the Shield of Arrav quest, the player must combine both parts of the broken shield to get the complete shield. One player must join the Black Arm Gang and retrieve it from upstairs in the base. The other player must be in the Phoenix Gang and must get it from the warehouse.
Despite the name, it is only a half of the shield, rather than one split shield.