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Item which is used with the Mysterious Potion to unlock end-game content.

Yellowletter.jpg

After using the Mysterious Potion on the Yellow Letter, you will obtain the recipe for Mysterious Engine.

Where to Find

After you have 100% completion of the Book, go to the last page and on the bottom right side of the pile of items, you will see a small yellow envelope. Tap on it to obtain the Yellow Letter.