Armour of Vulnerability

Armour (plate) (requires attunement)

Rarity: Rare

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

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

Source: Player’s Handbook