Instantáneo
Hasta una criatura objetivo obtiene -X/-X hasta el final del turno. Si X es 5 o más, regresa una carta de criatura con valor de maná de X o menos de tu cementerio al campo de batalla girada.
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.