Players play a Magic subgame, starting at 1 life and using their libraries as their decks. For the rest of the main game, if a source would deal damage to a player who didn't win the subgame, it deals double that damage to that player instead.
To play a Magic subgame, all players start a new game, using their libraries as their decks. No other cards, tokens, emblems, or anything else from the main game are included in the subgame.
If the subgame is a draw, no player won the subgame, so damage dealt to any player in the main game will be doubled.
If a player has fewer than seven cards in their main-game library, theyâ€™ll lose the subgame as it begins, regardless of how many mulligans that player takes.
After the subgame ends, all players shuffle their cards from the subgame to reform their main-game libraries, and they return to the main game from the point they left off, with The Countdown Is at One finishing resolving.