Source: Critical Role: Call of the Netherdeep

Wondrous item, common

When you would be hit by an attack, you can use your reaction to increase your AC by 5 until the start of your next turn, including against the triggering attack. You must be wearing the medal and able to see the creature that made the triggering attack to use this property. Once this property has been used, it can’t be used again, and the medal becomes nonmagical.