The Countdown Is at One

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 a player has fewer than seven cards in their main-game library, they’ll lose the subgame as it begins, regardless of how many mulligans that player takes.

If the subgame is a draw, no player won the subgame, so damage dealt to any player in the main game will be doubled.

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.