Whenever a creature you control attacks, defending player loses 1 life and you gain 1 life.
7/13/2016 If a creature attacks a planeswalker, the defending player is the player who controls that planeswalker.
7/13/2016 In a multiplayer game, each instance of the ability affects only one defending player. For example, if you attack player A with two creatures and player B with two creatures, each of those players will lose 2 life, not 4.
7/13/2016 In a Two-Headed Giant game, Campaign of Vengeance’s triggered ability has one defending player of your choice lose 1 life. The team doesn’t lose 2 life.