Rule 810.7c

As the declare attackers step begins, the active team declares attackers. If an effect of an object controlled by a defending player prohibits a creature from attacking him or her, that creature can’t attack the defending team. The active team has one combined attack, and that set of attacking creatures must be legal as a whole. See rule 508.1. Example: One player in a Two-Headed Giant game controls Teferi’s Moat, which says “As Teferi’s Moat enters the battlefield, choose a color.” and “Creatures of the chosen color without flying can’t attack you.” Creatures of the chosen color without flying can’t attack that player’s team.

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