Knight of Obligation | Magic: The Gathering
Formats Knight of Obligation is Legal in
Official Oracle Text for Knight of Obligation
Extort (Whenever you cast a spell, you may pay . If you do, each opponent loses 1 life and you gain that much life.)
Official Rulings for Knight of Obligation
4/15/2013 : The amount of life you gain from extort is based on the total amount of life lost, not necessarily the number of opponents you have. For example, if your opponent's life total can't change (perhaps because that player controls Platinum Emperion), you won't gain any life.
4/15/2013 : The extort ability doesn't target any player.