Nahiri states "put it onto the battlefield". Does that use the stack? If I have Hallowed Moonlight in hand, when do I play it? If I play it in response to Nahiri's ability, my opponent will just fetch a different creature. (They will have "wasted" Nahiri's -8, but I want more than that! I want to exile Emrakul!)
If I wait until they get Emrakul and that card gets put on the battlefield, am I too late to play Hallowed Moonlight?
The entire ultimate has to go off at once and can't be broken up. Once you find out what creature it is it is too late to exile it. You'd have to announce your response in response to the planes walker activation and then, yes, your opponent can now decide their search choice.
The latest you can play Hallowed Moonlight is before Nahiri's ability resolves. Your opponent can then choose to search for a different creature, or even no creature at all. Any time a player is searching for a card with some quality (ie, anything other than "search for a card"), they're permitted to "fail to find" a card with the required quality.
Nahiri states "put it onto the battlefield". Does that use the stack? If I have Hallowed Moonlight in hand, when do I play it? If I play it in response to Nahiri's ability, my opponent will just fetch a different creature. (They will have "wasted" Nahiri's -8, but I want more than that! I want to exile Emrakul!)
If I wait until they get Emrakul and that card gets put on the battlefield, am I too late to play Hallowed Moonlight?
Two Score, Minus Two or: A Stargate Tail
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