Comprehensive Rules
506.2a
During the combat phase of a multiplayer game, there may be one or more defending players, depending on the variant being played and the options chosen for it. Unless all the attacking playerâs opponents automatically become defending players during the combat phase, the attacking player chooses one of their opponents as a turn-based action during the beginning of combat step. (Note that the choice may be dictated by the variant being played or the options chosen for it.) That player becomes the defending player. See rule 802, âAttack Multiple Players Option,â rule 803, âAttack Left and Attack Right Options,â and rule 809, âEmperor Variant.â