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Guest 6801328
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PCs eventually return, defeat the invading force, and restore balance. Perhaps the adventure even involves them finding a way to reverse time before the billions die.
Again, the adventure continues. Try again if you want.
How is that different from having resurrection cast on a dead PC? How exactly are you defining "irreversible"?
EDIT: Ok, they fail to save the NPC princess and she dies. Does that meet your very narrowly defined criteria?