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Destroy target nonblack creature if its toughness is less than or equal to the number of cards in your graveyard.
Because Ghastly Demise isn't put into your graveyard until it's finished resolving, it won't be counted when determining whether the creature is destroyed.
Ghastly Demise can target any nonblack creature. Compare the creature's toughness to the number of cards in your graveyard as Ghastly Demise resolves to see if that creature is destroyed.
