The London Mulligan
For Mythic Championship II in London, we're going to be trying out a new mulligan rule that we have been playtesting internally for some time. We believe the new rule smooths out opening hand decisions even more, though it certainly has some implications for formats like Modern.
The rule we'll be testing in London is as such: When you mulligan for the Nth time, you draw seven cards, then put N cards on the bottom of your library in any order.
So, for example, let's say you're taking your second mulligan of a game, what we often call a mulligan to five. You would draw seven cards, select two, and place those two on the bottom of your library in any order. Then you would decide whether to keep or mulligan again.
While we have been testing this mulligan rule internally for a while, we are treating this tournament as a test. Once our game designers have reviewed the tournament, spoken to players, and looked at the data, we'll decide whether to implement the mulligan rule wider.
Ian Duke added the following on 2/26.
Since the announcement of the London mulligan test, we've received a number of questions about the specific interaction with the card Serum Powder. As noted in the official rules for the mulligan, putting cards from your hand on the bottom of your library is the last step of completing the mulligan before deciding whether to keep or take another mulligan. At the point where Serum Powder checks whether you could mulligan, the number of cards in your hand will already have been reduced by one for each previous mulligan you'll have taken.
For example, if you mulligan twice, then the third set of 7 cards that you see contains a Serum Powder, you'll first have to put two cards from your hand on the bottom of your library. Then (assuming you still have the Serum Powder in your hand), you can choose to exile your hand and draw five new cards, mulligan again, or keep your hand.
This is consistent with how Serum Powder has always functioned and with what we believe to be the spirit and intent of the card. Serum Powder was printed after the adoption of the Paris mulligan, so successive mulligans have always reduced the number of cards Serum Powder lets you draw. The same will be true under the London mulligan.
In practice, we expect players to shortcut the process of putting cards from their hand on the bottom of their library if they intend to mulligan again, since it typically won't matter outside of Serum Powder's specific interaction. That will be an acceptable shortcut at any level of rules enforcement.
Below is the official wording of the rule during the test.
103.4. Each player draws a number of cards equal to their starting hand size, which is normally seven. (Some effects can modify a player’s starting hand size.) A player who is dissatisfied with their initial hand may take a mulligan. First, the starting player declares whether they will take a mulligan. Then each other player in turn order does the same. Once each player has made a declaration, all players who decided to take mulligans do so at the same time. To take a mulligan, a player shuffles the cards in their hand back into their library, draws a new hand of cards equal to their starting hand size, then puts a number of those cards onto the bottom of their library in any order equal to the number of times that player has taken a mulligan. Once a player chooses not to take a mulligan, the remaining cards become the player’s opening hand, and that player may not take any further mulligans. This process is then repeated until no player takes a mulligan. A player can’t take a number of mulligans greater their starting hand size.