Defeat a God was one of the three "Horde Decks" created by Wizards during the original Theros block. It was designed for groups of players to play with sealed decks, and was likely inspired by the fan-created Zombie Horde EDH format. It's a cooperative format where a team of players faces down a deck that runs itself. The game works exactly as a normal game, except for these rules:
The players win if Xenagos Ascended is removed from the battlefield. Xenagos Ascended cannot be removed from the battlefield as long as a Reveler is on the battlefield. (This means that if Xenagos Ascended and one or more Revelers are on the battlefield and a spell like Wrath of God is cast, Xenagos will remain in play. However, if enough damage is assigned to Xenagos and all other Revelers, Xenagos Ascended will still die from lethal damage after all other creatures die.)
Players don't attack the Reveler deck, but attack individual creatures the Reveler deck controls. Each attacker can only attack one creature in a combat phase, but multiple attackers can attack the same creature. Both tapped and untapped creatures can be attacked.
The players share a turn instead of taking separate turns.
The players share a life total. Some of the Revelers' spells cause the players to take damage multiplied by the number of players. (E.g. Xenagos's Strike would deal 12 damage to the life total of a three-player team.)
The players go first and don't draw on their first turn.
The Reveler deck consists only of creatures, enchantments, and sorceries.
If any of the Revelers' permanents would leave the battlefield and go to any zone other than the graveyard, they die instead.
If the Reverlers player must make a choice, the players make the choice instead.
When the Revelers player would be dealt damage or lose life, instead deal that much damage to one of the Revelers or to Xenagos.
Ignore effects that are impossible for the Revelers player. (E.g. drawing/discarding cards or gaining life.)
At the beginning of the game, the players choose the number of Rollicking Throng creatures that begin on the battlefield. (Xenagos Asended also starts on the battlefield as well.)
During the Revelers' turn:
They untap all their creatures.
They the top two cards of their library on the stack.
They move to combat and attack, but only with creatures that are made to attack.