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If "Mirror Wall" is already face-up on the field when a monster declares an attack, "Mirror Wall" reduces the ATK of the attacking monster by half when the attack is declared.
If "Mirror Wall" is activated during an attack (including during the Damage Step), then it reduces the ATK of the attacking monster when "Mirror Wall" resolves. (Even though "Mirror Wall" normally reduces the attack by half when the attack is declared, it will still reduce the attack by half if you activate it after the attack is declared.)
If "Mirror Wall" halves the ATK of a monster, it remains halved as long as "Mirror Wall" remains face-up on the field. If "Mirror Wall" is removed from the field, the ATK of the monsters is restored.
If a monster attacks twice (over 2 turns, or if "Black Luster Soldier – Envoy of the Beginning" attacks twice in a row, etc.), its ATK is still only reduced by half once. So "Black Luster Soldier – Envoy of the Beginning" will have 1500 ATK for both of its attacks, for example.
If you have 2 copies of "Mirror Wall", and a monster attacks, its ATK is one-quarter. So "Blue-Eyes White Dragon" would have 750 ATK, for example. If 1 of the "Mirror Wall" copies leaves play, then the ATK is increased to half (if both copies leave play, the ATK is restored to full.)
You can activate "Mirror Wall" during the Damage Step before damage calculation.
The effect of "Mirror Wall" does not target.
"Reverse Trap" will have no effect on "Mirror Wall".
"Mirror Wall" may be activated in the Damage Step.
"Mirror Wall's" effect affects an affected monster until "Mirror Wall" is no longer on the field or is negated.
Payment to keep "Mirror Wall" on the field is optional.