My Body as Shield

You have dedicated yourself to the defense of others, even if it means you take the hit instead.  As a reaction, you can choose a creature within 5ft of you that is being attacked by a creature you can see.  This must chose to use this reaction after the attack roll is made, but the damage has not been revealed yet.  When the reaction is used, you become the target of the attack.  (The original attack roll is made against your AC instead).

Alternativly, when both you and an ally that is within 5ft of you are subjected to an attack that requires a save DC (i.e. fireball or lightning bolt), you may forfeit your save to give your ally advantage on the roll.