Stand Behind Me

You become an impenetrable Bulwark for your allies. When you see a creature hit an ally within 15 feet of you with a melee attack, you may use your reaction to move within 5 feet of your ally that is being hit. You take the damage in their stead, even if the attack roll would be below your AC. All resistances and immunities are taken into consideration as if the attack had hit you in the first place. 

You can use this feat a number of times equal to your proficiency bonus. You regain all uses on a short or long rest.