Obsidian

Obsidian is a realm outside of the Magic Dimension and its gate is called the Obsidian Circle.

Overview
According to Faragonda, Obsidian Circle is said to be the secret entrance to a dark and terrible place, a black dimension of ultimate fear and hate, and that the gate of the Realm of Obsidian is what keeps evil distant, adding that this was the place where the Ancestral Witches were generated. When Bloom said that maybe her parents were there, Faragonda replied that it was preferable to hope otherwise as anyone who enters Obsidian is hopelessly lost, corrupted by absolute evil.

According to Flora, Obsidian is the nightmare realm where monsters and the most formidable creatures are all caged, and Faragonda remarks that it is the story every child of Magix hears when they misbehave.

Season 3
In "Witches' Revelation,” when Bloom enters the Crypt of Cloud Tower, a place where the essence of the Ancestral Witches still remains, the Ancestral Witches talk of Obsidian, telling Bloom that her parents are kept there, in what they referred to being a dimension "far, far away", and it is a place that cannot be reached by magic.

Secret of the Lost Kingdom
The dimension first appears in The Secret of the Lost Kingdom. It is described as the realm of ultimate evil and darkness. Evil creatures are held captive in this dimension. In a sea of solid rock is where all the inhabitants of Domino were captured.

On top of a rock, Oritel stood and turned to stone. He tried to get his sword, the sword that Marion became one with to be with her husband forever. Behind are three caves with faces, in which the Ancestral Witches lived. Obsidian also has a guard, Mandragora, and its gate can only be opened by a key which is found in the Tree of Portals in Pixie Village.

If someone who is not evil enters Obsidian, they are lost in the power of evil. When the Winx, without Musa, enter Obsidian, they all except for Bloom, get lost. The four fairies then get to see their biggest fear: Aisha gets attacked by her Morphix, Flora gets attacked by dead plants, Tecna gets a headache because her brain receives too much data with no logic at all, and Stella disappears, meaning no one will being able to appreciate her beauty. When Obsidian is destroyed, the witches are set free, and at the end of the movie they fly through a hall to the Trix, saying that they are in need of younger bodies.

Trivia

 * Obsidian is a naturally occurring volcanic glass which of usually dark color which has been widely used throughout history to make blades and other cutting instruments.