Pox is a rare Sorcery card from Mystery Booster 2 (MB2), collector number 185. Current market price: $1.32 (foil $14.56), TCGplayer market prices via Scryfall.
Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control of their choice, then sacrifices a third of the lands they control of their choice. Round up each time.