Asfodelia

Asfodelia is a character featured exclusively within the Winx Club Comic Series. She never makes a physical appearance but debuts in Issue 107: The Dark Sirens, where Queen Niobe was kidnapped by the titular Sirens to become Asfodelia, the fifth and final Siren.

Appearance
Since Niobe never becomes Asfodelia, her appearance as a Siren remains unseen.

#107: The Dark Sirens
Asfodelia is first mentioned in Issue 107 by Limes to Niobe. Unaware of the Winx spying on the Sirens within the Deep Blue, Limes demands that Niobe forget her life and surrender herself to the darkness of the abyss. Niobe naturally refuses but Limes tells her that she still has no choice in the matter. She then goes on to explain the ritual of birthing a Dark Siren: Niobe will be lowered into the darkness of the deepest parts of the Deep Blue, where she will be forcibly turned into a Siren and emerge as Asfodelia with no memory of her past life as Niobe.

Magical Abilities
Though Asfodelia is never seen, it is safe to assume that she would possess magical powers common amongst the other four Sirens, like their dark magic and ability to use all forms of water to travel.

Trivia

 * Since Asfodelia would have been Niobe's new identity if the ritual were completed, she would be the only one of the Sirens whose past life would have been known.