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Up to one target creature gets -X/-X until end of turn. If X is 5 or more, return a creature card with mana value X or less from your graveyard to the battlefield tapped.
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.
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.
