A player messaged me today with a question asking me how to forward it to a high level judge. Apparently, he had asked 8 judges so far and they've all given him differing answers, so I decided I should bring it here.

Dredge player on the play. I want to mulligan using Serum Powder. Can my opponent look at my serum powder before choosing to mulligan or does he choose first to mulligan then I resolve my powder

Apparently, judges have told him that decisions are made, and then Serum Powder's effect takes place (which is what I have been told a few times before), and other judges have told him that the Serum Powder effect happens before mulligans are chosen.

Here are the pertinent part of the Comp Rules I've found:

103.4 …First, the starting player declares whether or not he or she 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…

103.4b If an effect allows a player to perform an action “any time {that player} could mulligan,” the player may perform that action at a time he or she would declare whether or not he or she will take a mulligan. This need not be in the first round of mulligans. Other players may have already made their mulligan declarations by the time the player has the option to perform this action. If the player performs the action, he or she then declares whether or not he or she will take a mulligan.

It seems to me that if the player with Powder is playing first, she or he uses Serum Powder before the opponent decides if they want to mulligan, so the opponent can look at the exiled cards before making a decision. If the player with the Powder goes second, the first player would have to decide before learning about the Powder.

Can I get confirmation if this is correct or not? If it is not, I'm curious as to how it actually works and why it works differently.

Edited Christopher Rumore (Aug. 25, 2015 10:01:43 AM)

You choose whether or not to use the Powder at the time when you're taking mulligans. If you use the Powder, right after you're done using the ability, that same player decides whether or not to mulligan. Since the cards are exiled face up, if the starting player uses the Powder, the other player will know what cards were exiled by the Powder's ability at the point when they determine if they want to mulligan or not.

Nathan Long
Wizards.Com Boards NetRep