Mutual Destruction

The last time that the game checks whether you have permission to cast a spell is before costs are paid. If you sacrifice a creature with flash to cast Mutual Destruction and then have no other permanents with flash, you’ll legally finish casting Mutual Destruction even though it no longer has flash. In this case, abilities that trigger whenever you cast a spell with flash won’t trigger.

You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.