You conjure a barrier of magical force in front of you that flickers with lightning.  While this ward is active, you have resistance to bludgeoning, piercing and slashing damage, and immunity to lightning damage.  Any time a creature hits you with a melee attack, you can use your reaction to have the ward deal 10 lightning damage to them.  This lightning damage comes from a pool of 50 lightning damage, and when this pool is gone the spell ends.  Whenever you are hit with any attack dealing lightning damage, you can use your reaction to absorb the lightning.  Your lightning pool then gains 10 more damage.