A Mystery Chest is a buff chest that can be opened after clearing Stage 81.

When opened, it provides a Dice or a buff tile and is reset when the Stage is cleared. If you proceed without opening the Mystery Chest, you cannot place a Dice on the tile where the Mystery Chest is located.

You can open the Mystery Chest for free, and it can be opened up to 3 times a day.
If it's not the Mystery Chest you want, you can open another chest by watching an advertisement.