The Sirenix Book is a book introduced in Season 5.
It has a burgundy cover with silver designs in the corners and a gold crescent moon in the middle.
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At first, they could not open it, until the Winx won the challenge in Graynor, and gained the Nature's Boon to unlock the book and open it. The Sirenix Book gave the Winx the Sirenix Boxes and their Harmonix powers. It also gave the Winx Club a warning about the curse that was bestowed upon the power; if the Winx did not complete the Sirenix quest in one lunar cycle, they would lose their powers forever.