The Chaos Keeper is a rare Legendary Creature — Elemental card from Unknown Event (UNK), collector number RZ02c. Current market price: $74.38, TCGplayer market prices via Scryfall.
The Chaos Keeper enters with a sealed Magic booster pack of your choice under it. (You provide a new pack each time.) Whenever The Chaos Keeper deals combat damage to a player, reveal its cards and put one of them into your hand. (Don't open the booster pack until the first time this ability resolves.)