Leo vs Michel: A Yu-Gi-Oh Duel

Why couldn't Luna be the savior in this duel?

Hollow Ghost:
Effect:
This card cannot be Normal Summoned or Set. This card cannot be Special Summoned except with "Hollow Spirit". Each time a card(s) is sent to the Graveyard, inflict 600 damage to your opponent. If "Hollow Spirit" is not in your Graveyard, the ATK of this card becomes 0.

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