The Black Circle (or the Dark Circle) is a magical item that debuts in Season 4.
The Black Circle is magically opposed to the White Circle and was created during the Great Fairy Hunt to oppose Earth Fairies' magic. According to Ogron, it is the source of the Fairy Hunters' power. When the Wizards captured Earth Fairies and plucked their wings, they also stole their power which was absorbed into the Black Circle.
It is a black circle with four black horns along it. It emanates a purple glow and can create a purple-black sphere around itself. The Black Circle can be shrunk to ring-size via a magic spell.
During the Wizards of the Black Circle's conquest to steal all magic on Earth by stripping the Earth Fairies' magic, they created the Black Circle to resist the combating force of the Earth Fairies' magic.
In "A Virtual World," it is seen when the Winx used their Tracix power on the White Circle to look into its past. There, they learned that the Black Circle is magically opposed to the White Circle and was created as a way to combat the power of Earth Fairies.
In "Sibylla's Cave," when the Wizards of the Black Circle began to weaken due to the growing belief in magic, they appeared before the Winx asking for their assistance. Ogron decided to surrender the Black Circle, where all of the Wizards' powers are contained. Anagan proposes to give the circle to Morgana as a sign of surrender. Bloom takes it and shrinks to ring-size to keep it in her presence.
The Winx decide to help them and chose Sibylla's cave as their safe haven. Sibylla is the most pragmatic choice, for as the Fairy of Justice she must remain impartial to all and will therefore protect all who come to her. Also, her caves prohibit Earth Fairies from entering, making it the perfect place for the Wizards to seek refuge.
In "Bloom’s Trial," it appears on Bloom’s finger throughout the episode. At Aurora’s Ice Tower, as it glows on Bloom’s finger, Morgana states that she must be in possession of the circle, but only after Ogron relinquishes it to her as a sign of submission during the trial of the Wizards of the Black Circle. Later, at Sibylla’s Cave, Bloom discusses the terms of the circle’s surrender with Ogron.
In "The Day of Justice," in Sibylla's cave, Bloom hands Ogron the circle to give it to Morgana during their trial. The Winx and the wizards head through a portal to Tir Na Nog and the wizards to await their judgement. The Black Circle is placed in a chest for safekeeping until the Wizards of the Black Circle are brought to trial. It is revealed by Duman, who has gone mad due to his unstable magical energy, that when Morgana places the Black Circle on her finger, all Earth Fairies will be destroyed. When the Ceremony of Truth begins, Morgana places the circle on her finger, activating a dark abyss that attempts to suck all Earth Fairies into it. However, this is prevented at a great cost as Nabu sacrifices all his energy to close the abyss, saving the Earth Fairies. He perishes in Aisha's arms.