Perfect Parry Riposte

Once per turn cycle, If you are equipped with a melee weapon or shield in your off hand, and a melee weapon in your primary hand, when targeted by a pierce/slash/striking attack within 5ft without advantage that succeeds, you can use your reaction to roll a saving throw of your off hand proficiency and add it to your AC.  If successful, you parry the attack breaking the opponents stance allowing for a guaranteed critical attack using your primary hand.  If failed your opponents attack will in turn strike critical.