A Challenge Deck is preconstructed theme deck with special Magic: The Gathering cards. It is a self-running deck with its own set of rules, also known as "player vs. environment". The challenge cards have special card backs, which means they aren't counted as regular Magic cards. When playing against the challenge deck players may use hero cards.
Playing against challenge decks is a casual multiplayer variant of Magic, much like Horde Magic. The players each have 20 life and take their turn simultaneously. Players may begin the game with up to two hero cards in their command zone to assist in defeating the challenge deck. The players win when they meet the challenge deck's win condition. The challenge deck is victorious if all of the players have been eliminated from the game.