If a creature can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having a creature attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.
16-03-2018
wotc
The controller of each attacking creature still chooses which player or planeswalker that creature attacks.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.
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: Creatures your opponents control attack this turn if able.
Yeong-Hao Han2018-03-16
