Bloodprice Half-Plate 2014

When a melee attack would hit you while you are wearing this armor, you can use your reaction to increase your Armor Class by up to 10 against that attack. If you do so, you lose hit points equal to 5 times the bonus you want to add to your AC. For example, if you want to increase your AC by 2 against that attack, you lose 10 hit points.