You can't cast this spell unless an opponent lost life this turn.
Creature spells you cast costless to cast for each 1 life your opponents have lost this turn.
10/1/2012 Remember, damage dealt to a player causes that player to lose that much life (unless the source has infect).
10/1/2012 Rakdos can be put onto the battlefield by another spell or ability even if no opponent has lost life that turn.
10/1/2012 Rakdos’s last ability cares about the total life lost, not necessarily what an opponent’s life total is compared to what it was at the beginning of the turn. For example, if an opponent loses 5 life and then gains 10 life in a turn, creature spells you cast will cost less to cast.
10/1/2012 Rakdos’s last ability can’t reduce the colored mana requirement of a creature spell you cast.
10/1/2012 If there are additional costs to cast a creature spell, such as a kicker cost or a cost imposed by the rules of a Commander game, apply those increases before applying cost reductions.
10/1/2012 Rakdos’s last ability won’t reduce the cost to cast Rakdos itself. It applies only to creature spells you cast once Rakdos is on the battlefield.