Melusina

The Melusina is a magical creature that appears in the Winx Club Comic Series.

Overview
The Melusina is a half-woman, half-serpent creature who roamed Andros before it became covered in water. They constantly fought against the mermaids. The last Melusina lived in the ruins of an ancient city on a remote island.

Season 6
To test his courage and readiness to protect Aisha, King Teredor sends Nex to destroy the last Melusina. When he does not return by the next day, the Winx fly to her island and discover she has captured him. She uses her wand to absorb some of Bloom and Tecna's magic energy. Her goal is to gather enough power to dry up Andros's oceans, reverting the realm to how it was when her kind ruled the land.

After Aisha frees Nex from the Melusina's chains, the two of them hatch a plan to destroy her. He believes the creature's weakness is water, which is why she wants to drain the oceans. Thus, Aisha is the only one who can defeat her.

Aisha decides to conjure a giant wave. Nex holds the Melusina back to give the fairy time to cast the spell. It turns out she does not have enough power, so the Winx send her some of their magic energy. She finally succeeds. The wave drowns the Melusina and sinks the island.

Magical Abilities
The Melusina from "Nex's Test" carries a wand that can absorb magic energy, as well as cast spells. It is not stated if all Melusines had these wands, or if this Melusina somehow acquired one.

Weaknesses
The Melusina is weak against water. Presumably, this is how they became extinct, after Andros became an ocean realm.

Trivia

 * According to European folklore, Melusine (or Melusina) was a legendary spirit of fresh water or a fairy. Melusine was half-snake from her waist down, and she married a nobleman only if he swore an oath to her to never look at her in her bath where she reveals her half-snake form. Melusine left the nobleman and flew away when he broke that oath.