Armor of Vulnerability

From Dungeon Master's Guide, page 152. Found on Magic Item Table G.

Description
Armor (plate), rare (requires attunement).

While wearing this armor, you have resistance to one of the following damage types: bludgeoning, piercing, or slashing. The DM chooses the type or determines it randomly.

Curse. This armor is cursed, a fact that is revealed only when an Identify spell is cast on the armor or you attune to it. Attuning to the armor curses you until you are targeted by the Remove Curse spell or similar magic; removing the armor fails to end the curse. While cursed, you have vulnerability to two of the three damage types associated with the armor (not the one to which it grants resistance).