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{2}{U}, {T}: Each player mills three cards. Exile up to two creature cards put into graveyards this way. Create an X/X blue Zombie creature token, where X is the total power of the cards exiled this way.
“Exiled this way” refers to the exile instruction that's part of the ability's effect. If the cards each player would put into their graveyard are exiled because of a replacement effect, those cards won't be in the graveyards to be exiled by the next part of the ability. In this case, the value of X will be 0. You'll put a 0/0 token onto the battlefield, then that token will die and subsequently cease to exist (unless something else raises its toughness above 0).
