Tritannus raids King Neptune’s underwater palace and transforms his whole mer family into mutants. Then, thanks to the dark magic of his trident, he manages to absorb all of the magical strength and energy of his father, King Neptune’s, mystical sword. Meanwhile, the Winx Club get a second clue to Sirenix which leads them to Musa's home world, Melody, busy searching for the second gem, the one of Empathy. During a fight with Tritannus, Musa is hit by a powerful dark spell and transforms into a terrible monster, bent on destroying the Winx. The Trix arrive with Tritannus on daphne`s doorstep and force her to tell them about Sirenix