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.)
Illus. Matt Thompson
MYTHICSPOILER.COM THE CYCLICAL MTG SPOILER
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KHANS OF TARKIR 
COMMANDER 2014
FTV: ANNIHILATION 
SPEED vs CUNNING 
MAGIC 2015 
CONSPIRACY 
MORE SETS
