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Exile all creatures you control, then reveal cards from the top of your library until you reveal that many creature cards. Put all creature cards revealed this way onto the battlefield, then shuffle the rest of the revealed cards into your library.
All creatures you put onto the battlefield with Mass Polymorph enter the battlefield at the same time.
Any abilities that trigger during the resolution of Mass Polymorph (such as a creature’s enters-the-battlefield ability) will wait to be put onto the stack until Mass Polymorph finishes resolving. The player whose turn it is will put all of their triggered abilities on the stack in any order, then each other player in turn order will do the same. (The last ability put on the stack will be the first one that resolves.)
If the number of creatures you exile is greater than the number of creature cards remaining in your library, you’ll wind up revealing your entire library, putting all creature cards revealed that way onto the battlefield, then shuffling your library.
The creatures you exile with Mass Polymorph remain exiled for the rest of the game.
