In Winx ClubEdit
This place was first introduced at the beginning of Season 3 when The Trix were sent there. Icy was able to free her sisters and herself, thanks to her power over ice. It was there where they first meet Valtor, the main villain in Season 3.
To escape, they broke open the portal which was linked to Andros, Aisha's home planet. Rumors spread that if the portal didn't close soon, Andros was going to be destroyed, because the energy of the Omega Dimension and that of Andros are of opposite nature.
The Winx arrived to close it, but it was Tecna, who went inside the vortex generated by the portal, and closed it, earning her Enchantix, but also falling inside in the process. The Winx believed that their fellow friend was dead, but Timmy trusted his instincts, which told him that Tecna was alive.
It was later revealed that she was stuck in the Omega Dimension. When the Winx and The Specialists go there to find Tecna, it's revealed that some prisoners who escaped their ice cells were ruling the planet. Thanks to Bloom, who lead the guard snake towards the prisoners, they were once again frozen. After the Winx leave, no more about the Omega Dimension was heard in that Season. According to Bloom, the frost/guard snakes maintain the magical balance of the Omega Dimension.
At the end of this season the Omega Dimension is where the Winx and the Wizards have their final battle, and also where the Wizards of the Black Circle met their end. After being defeated for the final time by the Winx and Nebula, Ogron, Anagan and Gantlos are frozen themselves there, never to be seen again after they fall in a ravine, when the ice around them crumbled because of the rage of the final battle.
Nebula wanted to remain there as a punishment to herself, because she thought that her wish for revenge against the Wizards had made her like them, but was convinced by Bloom that she was not evil and that there was still some light in her heart, and she returned with the Winx to Morgana, who forgave her. After this the Winx were reunited with Aisha and a ray of sunlight appeared there, upon which Brandon noted that sun rays are rarely seen in the Omega Dimension and that one appeared it was a special day.