Rule 826.4.b
If more than one unit with Backline is present with the same controller in Combat, damage may be assigned to any of them. Units with Backline are invalid assignments until all units without Backline have lethal damage assigned to them.
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If more than one unit with Backline is present with the same controller in Combat, damage may be assigned to any of them. Units with Backline are invalid assignments until all units without Backline have lethal damage assigned to them.