The Dark Dimension is a hidden dimension exclusive to the Winx Club Comic Series. The Lord of Shadows, Neruman, resides within the Dimension and has done so ever since his escape from the Fortress of Light when he was outed for practicing dark magic.
Dark Dimension Portal
The Dark Dimension can only be accessed through the use of a portal and there appears to be only one in existence. The location of the portal remained unknown until Issue 97 thanks to Tecna's magical tracking device, the Searchix. Balazar was able to force the portal open through the use of Morgana's Necklace and the portal remained open due to the necklace not having enough magic to reclose it. The portal itself is a large green, skull-shaped door, decorated by three smaller skulls that sit along the top of the skull door. It is also surrounded by darker green edges with a few golden stripes.
The Dark Dimension
The Dark Dimension is a dimension made entirely out of darkness and universal dark forces, making it perfect for Neruman when he was in search of refuge from his former colleagues at the Fortress of Light. Many different planets float around in the Dimension and it is unknown whether or not there is any natural life on any of them, or even if the Dimension itself is capable of housing its own life forms. Neruman's lair is located within the Dimension, specifically on a purple skull-shaped planet reminiscent of the portal. The Dimension is also capable of surviving after Neruman's defeat as it shows no signs of collapsing.
After escaping from the Fortress of Light, Neruman sought refuge within the Dark Dimension and built up his lair there. He has been residing within the Dimension for centuries, sending his shadows out to collect the suffering and anguish of life forms around the Magic Dimension to help him regain his body. His lair is first seen in Issue 86 and seems to mainly be comprised of a dimly light room with large glass windows and purple pillars with golden swirls. There is a pool of energy in the middle of the room and a light purple throne is located at the back of the room, where Neruman is always seen sitting. There are also other rooms within the lair, like the potions room that Balazar and another troll-like slave had to use to make Neruman a potion that would help regain his former body. The lair, alongside Neruman's shadows, collapsed upon Neruman's demise in Issue 97.