Enter the Dungeon is a rare Sorcery card from Unhinged (UNH), collector number 53. Current market price: $1.83 (foil $15.63), TCGplayer market prices via Scryfall.
Players play a Magic subgame under the table, starting at 5 life and using their libraries as their decks. The winner searches their library for two cards, puts those cards into their hand, then shuffles.