Rakdos, Lord of Riots

• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  Damage dealt to a player causes that player to lose that much life.
• WWizards of the Coast2020-11-10
  Official Ruling
  Rakdos can be put onto the battlefield by another spell or ability even if no opponent has lost life that turn.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  If there are additional costs to cast a creature spell, such as a kicker cost or the commander tax, apply those increases before applying cost reductions.
• WWizards of the Coast2020-11-10
  Official Ruling
  If an opponent lost life and subsequently lost the game, Rakdos's first ability is satisfied, and its last ability counts that loss of life.
• WWizards of the Coast2020-11-10
  Official Ruling
  Rakdos's last ability can't reduce the colored mana requirement of a creature spell you cast.

Rulings via Scryfall