A Good Thing is a rare Enchantment card from Mystery Booster Playtest Cards 2021 (CMB2), collector number 86. Current market price: $0.36, TCGplayer market prices via Scryfall.
Spells and abilities you control can't destroy, exile, target, or cause you to sacrifice A Good Thing. At the beginning of your upkeep, double your life total. Then, if you have 1,000 or more life, you lose the game.