Battle the Horde 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 Horde deck loses if there are no cards remaining in its library and controls no creatures.
When the Horde deck loses life, it mills that many cards. It otherwise does not have a life total.
The players share a turn instead of taking separate turns.
The players share a life total.
The players go first and don't draw on their first turn.
If any of the Horde's permanents would leave the battlefield and go to any zone other than the graveyard, they die instead.
Ignore effects that are impossible for the Horde player. (E.g. drawing/discarding cards or gaining life.)
If the Horde player must make a choice, the players make the choice instead.
At the beginning of the game, the players choose the number of spells the Horde casts each turn.
The Horde does not act on its first two turns. Don't click "Click to Start Horde's Turn" until its third turn.
During the Horde player's turn (after those first two):
The Horde untaps all their creatures (Minotaurs).
The Horde player casts the top cards of their library, the amount chosen at the beginning of the game plus the number of artifacts they control. (The ability to cast an additional spell is listed on each of the artifacts.)
The Horde player goes to combat and attacks. Any of the players' creatures can block the attacking minotaurs.