Harmonix is a temporary transformation given to the Winx Club in order to finish the Sirenix quest.
When the Winx girls learned that their Believix powers are less effective and stronger underwater, the Winx are on a quest to aquire the ancient Sirenix. The Sirenix Book gave the girls Sirenix Boxes. Each of the boxes contains their Guardian of Sirenix. They granted the girls to fight in land and underwater. Once the Winx completed the quest, the guardians opens the gate to the Infinite Ocean enabling them to obtain Sirenix and become Sirenix Fairies.
The Winx must find the three gems - the Gem of Self-Confidence, the Gem of Empathy, and the Gem of Courage. The girls must gain the boon of the Ancestral Spirit of Nature in order to open the Sirenix Book and recieve Sirenix Boxes.
The girls' hair are much longer and are tied, except for Tecna, which is short. The outfits consist of a dress which is either strapless or in Tecna and Aisha's case, shoulder strap, mini-skirts and a veil at the back of the skirts. The girls also wears seashell tiara's or jewelries, high-heeled sandals with two ribbons attached to the girls' legs. Harmonix also gives them multi-colored wings with wing effects.
Like the Believix spell effects, there is also Harmonix wing effects. It shows the girls their power signs such as hearts, flowers, stars, swirls, music notes, and diamonds.