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Até uma criatura alvo recebe -X/-X até o final do turno. Se X for igual ou superior a 5, devolva um card de criatura com valor de mana igual ou inferior a X de seu cemitério para o campo de batalha virado.
You can cast Black Sun's Twilight without a target (presumably with X equal to 5 or more) just to return a creature card. However, if you do choose a target, and that target is illegal at the time the spell tries to resolve, the spell won't resolve and none of its effects will happen. You won't get to return a creature card to the battlefield.
The creature card you return, if any, isn't a target of Black Sun's Twilight. You choose it as Black Sun's Twilight resolves if X is 5 or more.