Rakdos, Lord of Riots

Damage dealt to a player causes that player to lose that much life.

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 {5} less to cast.

Rakdos can be put onto the battlefield by another spell or ability even if no opponent has lost life that turn.