In Season 5, after learning that their Believix powers are less stronger and effective underwater, the Winx Club will go on a quest to find the ancient Sirenix transformation and magical power. To help the Winx, the Sirenix Book gave each of the Winx a Sirenix Box which contains their Sirenix guardian, and the Guardians of Sirenix granted the Winx the power of Harmonix that will allow them to fight in mid-air, on the land and under the water. Once the Winx did all the required tasks for Sirenix, they opened the gate to the Infinite Ocean thus enabling them to earn Sirenix. They eventually succeed in their goal and obtain Sirenix.
The hair mostly stays in loose ponytails and the girls wear seashell tiaras or seashell barrettes in their hair, heads or foreheads. The Harmonix outfit consists of a layered dress with a top that is either strapless or a strap that goes on only one shoulder, a belt, a mini-skirt and a layered cape attached to the skirt which resembles mermaids' tail. The girls wear heeled sandals with two ribbons attached around the feet and legs, and two long flowing ribbon strips attached to the sandals at knee level. The wings are in medium side, multi-layered and colorful with some special effects.
Since Believix is weak underwater, and the Winx must go underwater to find the three gems to earn Sirenix, they must gain Nature's Key, the boon of the Ancestral Spirit of Nature, to open the Sirenix Book and receive Sirenix Boxes, and their Guardians of Sirenix grant them Harmonix.
Harmonix allows the Winx to use their magic easier in mid-air, on the land and underwater. It also helps the Winx swim faster.
Just like in Believix, each Winx has different effect whenever her wings flutter.
Known Harmonix Fairies
- The name Harmonix comes from the word "harmony". And is probably derived from the fact that the Winx have to achieve Harmony in themselves via the Nature Boon before they can earn Sirenix.
- Harmonix is the third transformation to be first earned in the sixth episode of a season. The first and second are being Enchantix and Believix.
- The sandals in Harmonix look similar to the sandals in the Enchantix transformation.
- After it was leaked, the song's lyrics were changed and the transformation was re-animated.
- Harmonix may have the same strength of Sirenix. But due to the fact that Harmonix is temporary (the Guardians of Sirenix grant them this power to help them finish the Sirenix quest, and if they fail the quest, they will lose their powers forever), they need to acquire Sirenix to become stronger. They also need Sirenix to enter the Infinite Ocean because Harmonix does not give the ability to enter it.
- Bloom's and Tecna's final poses in Harmonix's transformation are the same.
- In the early concept art, Harmonix had gothic appearance, but was later changed. The final designs of Sirenix bears a strong resemblance to this concept.