Acheron

Acheron is a villain from Season 6 whom Selina serves.

Overview
As a dark wizard who learned immensely powerful dark magic on Earth from the terrestrial witches in order to become the greatest sorcerer in the Magical Universe, he created the Legendarium. But he couldn't control it and was trapped inside it. Now, he’s trying to break free with Selina's help. He is currently using Selina to release him in exchange for granting her magical abilities.

Appearance
Acheron has pale skin, glowing yellow eyes, a crooked pointy nose and a black goatee. He has black marks (which might be kohl) around his eyes with triangular black marks on its edges.

His outfit is similar to the Dark Witch outfits of the Trix, consisting of a thigh-length armor-like tunic with a hood and large hanging sleeves and baggy trousers tucked in boots. It is red with black marks, the part on his shoulders is pale grayish purple and it has a yellow ruby on his chest.

Season 6
In "The Golden Auditorium", After the Treants failed to take over Linphea College, it is revealed that Selina is in fact working under the orders of a mysterious dark being called Acheron housed inside the Legendarium.

In the "Attack of the Sphinx", Acheron tells Selina to find Eldora's Diary and destroy it before the winx do because it is the only link to telling how to seal the Legendarium for good.

In "The Fairy Godmother", Eldora tells the Winx the past about Selina and Acheron where she agrees to work under him. Then Acheron gives a bit of power to Selina, dubbing her the Witch of Snakes. Eldora comes in and tells Selina not to read from the Legendarium and warns her of Acheron as the sorcerer who created the book and wound up trapped in its pages. One night, Selina made her decision and pledged her loyalty to Acheron. She stole the Legendarium and fled the Forest of Flowers. Eldora fears that Selina is trying to free Acheron all this time.

In "Acheron", Acheron is freed when Selina uses the part of the Dragon's Flame that she stole from Bloom and fights Bloom one-on-one in the Legendarium World. Bloom uses the legend of Alfea against him and successfully traps him in a box given to her by Eldora.

Trivia

 * According to Greek mythology, the Acheron is one of the five rivers of the Greek Underworld. The spirits of the dead had to cross the Acheron on a boat to enter the Underworld.
 * The main Season 3 villain, Valtor, also wanted to become the greatest sorcerer of the Magical Universe.