This item is designed to encourage my paladin to act a little more paladin-y.
If you are a paladin, when you undertake a quest, in addition to whatever vows the GM says are required, take the following vow: Innocence (forbidden: to harm an innocent*). You take +2 armor until your quest ends. If you harm an innocent, your quest ends automatically and you take -1 forward until you redeem yourself. *GM defines “an innocent.” If you are not a paladin, the shield functions as normal.