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Any time you could mulligan and Serum Powder is in your hand, you may exile all the cards from your hand, then draw that many cards. (You can do this in addition to taking mulligans.)
You can use Serum Powderâ€™s second ability only while itâ€™s in your hand. If this card is in your hand, you can choose either to mulligan or to use Serum Powderâ€™s ability. Using the ability doesnâ€™t prevent you from taking further mulligans, and taking a mulligan doesnâ€™t prevent you from using a Serum Powderâ€™s ability if you happen to draw one in your new hand.
Unlike a normal mulligan, Serum Powder lets you draw the same number of cards that were in your hand, not one less. If you have already taken one mulligan and your hand contains six cards including a Serum Powder, you can choose to exile those six cards and draw six new cards.
The cards in your hand are exiled for the rest of the game; they arenâ€™t shuffled back into your library if you take another mulligan. Cards exiled are always face up unless the effect that exiled them says they arenâ€™t. Be sure to return those cards to your deck for your next game.