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Until end of turn, creatures you control gain menace and "Whenever this creature deals combat damage to a player, exile the top card of that player's library face down. You may look at and play that card for as long as it remains exiled, and you may spend mana as though it were mana of any color to cast that spell."
You must pay all costs to play cards exiled this way, and you must follow all normal timing permissions and restrictions. For example, if you play a land this way, you may do so only during your main phase while the stack is empty and only if you haven't yet played a land (unless another effect allows you to play additional lands).
