Phantom Dragon

 This monster's appearance is similar to Cloudian - Storm Dragons.

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Coincidentally, both monsters also made their OCG/TCG debut in the same Booster Pack (in this case, Light of Destruction.)

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OCG Rulings

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The "When your opponent Special Summons a monster (except during the Damage Step): You can Special Summon this card from your hand." effect is a Trigger Effect that is activated from the hand.: Phantom Dragon

This effect cannot be activated during the Damage Step.

The "2 of your Monster Card Zones cannot be used." effect is a Continuous Effect.

Even if you only have 1 unused Monster Zone, you can still Normal or Special Summon "Phantom Dragon". (When "Phantom Dragon" is Normal or Special Summoned, if you have 1 unused Monster Zone, that Monster Zone becomes unusable.) Also, if another Monster Zone later becomes available, it becomes unusable, resulting in 2 unusable Monster Zones on your side of the field.: Can "Phantom Dragon" be Normal Summoned or Special Summoned while you do only have 1 unused Monster Zone?

If the effect of "Phantom Dragon" is negated by "Skill Drain", the 2 unusable Monster Zones become usable. If the effect of "Skill Drain" stops appplying, the 2 Monster Zones become unusable again.: If "Phantom Dragon" is affected by "Skill Drain", what happens?

You can Pendulum Summon 2 "Phantom Dragon" while you control no monsters. When they are Pendulum Summoned, since you have 3 unused Monster Zones, the effect of the first "Phantom Dragon" makes 2 of them unusable, and the effect of the second "Phantom Dragon

Previously Official Rulings

You cannot activate "Phantom Dragon's" effect during the Damage Step.

If a face-down "Phantom Dragon" is flipped face-up, you will select 2 Monster Card Zones to make unusable. If none can be selected, the next 2 Monster Card Zones to become available will be made unusable.

"Phantom Dragon" can be Tribute Summoned if the Tributed monsters will create 2 Monster Card Zones that can be made unusable.The latest print of "Phantom Dragon" has Problem-Solving Card Text which does not include any requirement to have additional Monster Zones in order to Summon it.

2 "Phantom Dragons" cannot select the same Monster Card Zones to make unusable.

Once a Monster Card Zone is chosen, you cannot change your choice to a different Monster Card Zone later in the Duel.

If you have 1 face-up "Phantom Dragon" while "Skill Drain" is active, when your opponent Special Summons a monster you can Special Summon another "Phantom Dragon" into one of the Monster Card Zones that was made unusable by the negated "Phantom Dragon".

If you take control of your opponent's "Phantom Dragon", you will select 2 Monster Card Zones on your side of the field to make unusable. If you do not have any available, the next 2 Monster Card Zones to become available will be made unusable.

Out of Date

You cannot Summon "Phantom Dragon" if you do not have 2 Monster Card Zones that you can make unusable. However, "Phantom Dragon" can be Set or Special Summoned face-down when you do not have 2 Monster Card Zones to make unusable.