previous transformation: Hamonix
next transformation: Unknown
Sirenix is the second transformation of Season 5. This transformation is said to be water-related. The Winx needed this transformation to fight Tritannus and the Trix since their Believix powers doesn't really work on water. The source of this transformation is located in the Lake Roccaluce. When a fairy reaches this level, she can easily go to the Infinite Ocean without the help of the selkie's powers.
According to Miss Faragonda in Return to Alfea, the Sirenix power is an ancient power that was born of the magic of the ocean and it will make the Winx more powerful underwater since their Believix power is less effective under the sea. She also mentioned that no one had acquired Sirenix for a long time and that Daphne was the last fairy who had acquired it and that she was the only one who knew where the ancient Sirenix Book is hidden.
When Bloom asked Daphne about the location of the Book of Sirenix, Daphne told her not to look for it because when the Ancestral Witches attacked Domino, she acquired Sirenix and was ready to fight them by using it, but the Witches had placed an evil spell on Sirenix and when Daphne tried to use Sirenix it turned against herself and caused her to become a spirit without a body. Bloom ended by persuading Daphne, who reveals that the Sirenix Book is in the Magical Archives of Alfea.
The Source of the Sirenix power is found in the cave where Daphne dwells in Lake Roccaluce which grants Sirenix powers to Fairies who have completed the Sirenix Quest. It also protects the Sirenix powers of Sirenix fairies.
Also, Sirenix grants the Fairies and Trix access to the Infinite Ocean without absorbing all of the Selkies' powers like Tritannus did.
(Source: Winx Club Wiki http://winx.wikia.com/wiki/Sirenix#Overview)