Rakdos, Lord of Riots is a mythic Legendary Creature — Demon card from Return to Ravnica (RTR), collector number 187. Current market price: $1.60 (foil $9.77), TCGplayer market prices via Scryfall.
You can't cast Rakdos unless an opponent lost life this turn. Flying, trample Creature spells you cast cost {1} less to cast for each 1 life your opponents have lost this turn.