Restarting the Game
One card (Karn Liberated from the New Phyrexia expansion) restarts the game. When the game is restarted, it immediately ends without a winner or loser, and all players still in the game when it ended start a new game. The new game is a normal game in every respect, except that the controller of the spell or ability that restarted the game plays first, and that spell or ability may have additional instructions to follow as the new game begins. All cards that were involved in the restarted game are involved in the new game, even if they didn't begin the restarted game in their owner's deck.
Rules for restarting the game were added to the Comprehensive Rules in the rules update associated with the release of New Phyrexia. As Aaron Forsythe explains, "Riffing off the world-creation and time-travel ideas, I put forth the phrase 'Restart the game,' and the team took it from there."