Duelist

At the end of your turn, or when a creature makes a melee attack against you, you can use a reaction to enter a dueling stance against one creature until the start of your next turn. You must specify which creature when you enter the stance. While in a dueling stance, you can add your proficiency bonus to your AC against all melee attacks made by that creature as long as there are no other hostile creatures within 5 feet of you. You must be wielding a finesse weapon to gain this benefit.