For those who are still wondering what these are for:
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Voidwatch: Crystal Petrifact
Some of you might be finding Crystal Petrifacts on your adventures. As stated in the version update notes, this item will be required for the final chapter of Voidwatch, but currently cannot be used.
This item will be the equivalent of Voiddust once the final chapter is implemented. By exchanging a certain amount of Cystal Petrifact, you will be able to obtain trigger items to challenge the monsters of the final chapter.
That said, while it may just be taking up space in your inventory right now, you might want to hold on to them for future use. Unlike Voiddust, Crystal Petrifact can only be obtained under certain circumstances and is not designed to be sellable.
Hope you are all looking forward to the final chapter of Voidwatch!