The Dark Door

 This card might be a reference to the Door of Darkness that Yami Yugi constantly referenced in the first anime series.

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This card has a weaker anime-only Trap counterpart, Narrow Corridor.

OCG Rulings

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While the effect is being applied, only one monster can attack per Battle Phase.: Spell Card > The Dark Door

Previously Official Rulings

A single monster may still attack more than once while this card is on the field, such as "Hayabusa Knight", "Asura Priest", "Black Luster Soldier - Envoy of the Beginning", "Tyrant Dragon", "Ruin, Queen of Oblivion", a monster equipped with "Twin Swords of Flashing Light - Tryce" or a Spellcaster-Type Monster affected by "Diffusion Wave-Motion".

If the effect of "Weather Report" is used to gain an additional Battle Phase, one monster may attack once each Battle Phase.

Amazoness Archers: Monsters that have already attacked cannot attack again. If "The Dark Door" is active, your opponent still can only attack with 1 monster. The ATK reduction of "Amazoness Archers" is still applied in these cases where monsters cannot attack.

Berserk Gorilla: If "The Dark Door" is active and you control "Berserk Gorilla" and another monster, you can choose for your other monster to be the 1 monster that attacks.