Dark Dimension

The Dark Dimension is a hidden dimension mentioned in the third season of Winx Club but appears only in 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 practising 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.

Neruman's Lair
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.

Season 3
In Fury!, Valtor tells Bloom that he trapped Marion using a spell he learned from the rock cannibals of the Dark Dimension to trick Oritel into lowering his guard while trying to save her so Valtor could defeat him. This was later revealed to be a lie by Valtor, though whether rock cannibals do exist in the Dark Dimension or not was never clarified.

In The Crystal Labyrinth, when Valtor finds out that the Winx have gone to the Golden Kingdom to find powers that could defeat him, he gives Icy a box of Ancient Crow's Dust from the Dark Dimension and sends the Trix to destroy Bloom in the Veil Mist Forest.

Icy uses the Ancient Crow's Dust on Bloom, which renders her powerless temporarily, giving the Trix an advantage until the Winx return from the Golden Kingdom after obtaining the Water Stars and help Bloom defeat the Trix.

Season 4
A glimpse of the Dark Dimension is first seen in The Golden Reef, as it starts off with Neruman impatiently waiting for a potion said to help him recover by Balazar and another troll-like slave in his lair.

Using his shadows, he rushes the two troll-like creatures until Balazar hands him the potion he requested. However, the potion does not give Neruman his body back and he lashes out until Balazar explains that he has not absorbed enough negative energy for it to work. At that moment, one of his shadows returns having collected a substantial amount of negative energy from a realm called Iberian and, after merging with the shadow, Neruman begins to feel the potion's positive effects. Realizing that Balazar's words rang true, Neruman sends some of his shadows in the direction of Earth, as he believes that humans are weak and easily prone to suffering.

Unlike Issue 86, the Dark Dimension does not make any appearances in Issue 95. Instead, it is officially name-dropped by Ogron as he reveals everything concerning Neruman and Morgana's missing necklace to the Winx after he, Gantlos and Anagan were defeated and abandoned on Solaria.

Here, Ogron reveals that Morgana's necklace is currently in the hands of Balazar, Neruman's right-hand man, as he and his fellow Wizards of the Black Circle were tasked with giving it to him after they had stolen it sometime before the events of Issue 85. With it, Balazar plans to force open the portal into the Dark Dimension to free his Lord of Shadows, and he may have already done so!

Now that they know roughly about the necklace's whereabouts, the Winx vow to find the location of the Dark Dimension Portal to retrieve it and put an end to Neruman's dastardly plot. By Issue 97, the Winx, exhausted and stressed out from their search, finally uncover the location of the portal leading into the Dark Dimension hidden away within the Fortress of Light, where it remained undetected even by the monks, templars and wizards who worked and lived there. Their discovery of the Dark Dimension portal also allowed for the Winx to finally find Balazar, apprehend him and take back the necklace, however, they journey into the portal soon after as, according the Balazar, their only option for stopping the Lord of Shadows now is to defeat him before he emerges.

Once they enter the portal, the Winx are met with an endless sea of darkness with multiple seemingly-uninhabited planets. They quickly take notice of a small skull-shaped moon reminiscent of the portal and head for it, deducing that Neruman's lair may be located on it.

Just as Bloom had suspected, the Winx find Neruman's lair on the skull-shaped moon and burst into the throne room to find Neruman waiting for them. He then launches a surprise attack on the Winx, declaring them to be foolish for rushing into the Dark Dimension without being aware of the risks.

Trivia

 * A similar dimension, also called the Dark Dimension, appears in the fictional universe of Marvel Comics, where it is the residence of the villain Dormammu and of the Mhuruuks, a species of humanoid extra-dimensional beings naturally adept at using magic
 * In Season 3 Episode 14 Fury!, Valtor mentions a spell he learned from Rock Cannibals from Dark Dimension when he fought Marion