Pincushion
A named shield peppered with holes -- enough to be a a sieve.
Identify reveals it to be a +2 shield with advantage on saves against all spells that target the wielder.
Against any mundane or enchanted arrow, it negates all armor and magical defense bonuses for the wielder, and its own bonus becomes a negative.
AC = 10 - 2 (Pincushion) + DEX bonus + nonmagical (race/class) bonuses
If an enchanted arrow strikes the wielder and releases a spell, the wielder still gets advantage on the save. The shield's just terrible about arrows themselves.
Special note:
If the wielder is in the area of effect of Conjure Barrage, they take triple damage, no saving throw.
I had a simpler more mundane version of that idea.
Shield of Dynamic Deflection - This +1 shield gives advantage on Dex saving throws against area effects. All projectile attacks that pass within 10 feet of the bearer target them instead of their original target.