How do i make a procedure when the player or entity is attacked the mob or player that attacked the player.

 How do i make a procedure when the player or entity is attacked the mob or player that attacked the player something happens because i want to make explosive enchantment that does what it says and explodes the opposing player and i know how to check for enchantments and armor slots, but i just cant work out how to check for when the person is attacked it effects the mob or player that has attacked the person with the enchantment.