Winx Club Forever
As explained by Roxy, the Dark Guardian's purpose is to prevent anyone from finding and freeing the Earth Fairies who had been imprisoned within the island, sealed in their own kingdom.
It uses visions and dreams to lure its victims or to catch them off guard. Once it has captured them it traps them in a dungeon puts them in a deep sleep.
It can also appear in dreams and even affect it.
It appears as a moving elongated, tall, cylinder, somewhat like a sea anemone and is bumpy throughout its body. It is a deep, dark purple but is slightly translucent which can be seen when the Winx were inside it. The top part is tentacle-like but flows like hair when wind is present. At its base, a cloudy mist covers it and moves along with it. When it is stationary, the base appears to fold inward.
When wanting to appeal to its victims it can change its color to appear more human- and life-like. But it can also color its other selves in darker shades matching its normal appearance.
As a creation of the Wizards of the Black Circle it is imbued by their power. Its main ability is producing astral drops that takes on any shape or form; be it a mermaid or even a plant. These astral drops appear to be able to work independently. Because it uses visions and dreams to lure people it can be assumed that it can read or sense people's deepest desires. As such, it can appear in dreams or intercept telepathic communications and affect them. Such as taking Morgana away to prevent Roxy from getting too much help.
It is also very resilient to magic and strong; able to ram through Flora's Winter Roses. Destroying only its astral drops do not eradicate it completely but it does require time to recover after that.
With mermaids, it seems that it can generate hypnotic sounds.
In "The Enchanted Island" it first appears towering over Roxy's tent. When she steps out it breaks through Aisha's Morphix Barrier and captures her. It moves on to its next target, first by appearing as a harmless vine that spirals down next to Flora. When she least expects it, it snatches her from Bloom and Musa.
Its next victim is Aisha and it lures her into the ocean by posing as three mermaids. After she is captured, it moves on to Stella where it entices her as a handsome, muscular man in the bushes. When Tecna catches up to Stella, it reveals itself to be a centaur and then captures them both in its true form.
Finally, it moves onto the remaining two: Musa and Bloom. It easily sways Bloom as a unicorn which mystifies the fairy. But its presence alerts Musa who tells Bloom that it is not real. The Dark Guardian then captures its last victims. It then takes its kidnapped victims into a dungeon but returns when it realizes that they had escaped. It splits itself into four astral drops: the visions the Winx saw before they were captured.
While the others deal with it, Roxy and Bloom head for a room that has a panel carved out like the White Circle, indicating as a keyhole and the appropriate one must be found for Roxy's to activate and release the fairies. Noticing this, it returns again and avoids the other five fairies as Roxy and Bloom fly towards the correct room. It almost got into the room but was then eradicated when Roxy unlocks the panel, thereby freeing the Earth Fairies.