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If Optimus Prime - Battlefield Legend is at 13 damage counters and he attacks and flips into One Shall Stand One Shall Fall, and I assign 3 damage to him, causing him to KO, what happens to the ATK Damage? Or is there no attack and the turn ends? Does my opponent need to flip for defense?

In this scenario, the attack ends and you do not flip for defense.
Source: Wizards of the Coast (2019-05-02)    【Link to Ruling】
Tags: Optimus Prime, Battlefield Legend, Optimus Prime, Battlefield Legend