Darkar's fortress was an underground fortress which was difficult to enter, let alone approach, because of the intense negative energy found there, as well as the lack of sunlight, weakens creatures living on the surface and also because it was guarded by the Shadow Monsters of the Under Realm were under Darkar's command. Because of this even Bloom, despite possessing the Dragon's Flame, the most powerful power in the Universe, was weakened when she went to Darkar's Fortress to save the Pixies.
Although the fortress has been seen only in Season 2, it is unknown for how long has it been existing and whether Lord Darkar was already residing there before he was defeated by the Lord of the Knights of Roccaluce.
At the beginning of this season, Darkar kidnapped the Pixies and imprisoned them in his fortress in an attempt to find the location of the Codex. Aisha tried to rescue the Pixies but she was stopped and defeated by Lord Darkar, and she only managed to save Piff.
After Aisha arrived at Alfea, Faragonda sent her together with Bloom, Stella, Sky and Brandon to rescue the Pixies. After many difficulties and passing through the realm of Downland, they managed to save the Pixies with the help of a mysterious Paladin who was later revealed to be the new teacher at Alfea, Professor Avalon.
Towrds the end of the season the fake Avalon who had been sent to Alfea by Darkar kidnapped Bloom took her to Lord Darkar who turned her into Dark Bloom to use her to obtain the ultimate power of the Relix. The remaining Winx and Specialists then launched an assault on the castle and were soon followed by two of the school directors and a Red Fountain teacher (Saladin sent Codatorta in his place). After Darkar was defeated in the Relix Realm, most of Darkar's fortress was left in ruins.
Though Lord Darkar did not physically appear during Season 3, Stormy suggested to Valtor that they should move to Lord Darkar's fortress, Darcy rejected the idea because she does not want live "undergound like a mole."
- One of the creatures living in Darkar's fortress and serving Lord Darkar, the Spit Monster, has a similar appearance to the Xenomorph from the Alien film series and its ability to spit a slimy substance that recalls the water-like liquid/resin which flows from the Xenomorph's mouth.
- It was called as Darkar's Fortress/Darkar's Castle in the Cinélume version and Shadowhaunt in the 4Kids version.
- Lord Darkar's fortress might have been partly inspired from Barad-dûr, the fortress of the Dark Lord Sauron from The Lord of the Rings by J. R. R. Tolkien, and by Mount Doom, the volcano found near Barad-dûr.
- In The Lord of the Rings, Sauron had poured a large amount of his power into Barad-dûr and Mount Doom such that, when Sauron was defeated for good, Barad-dûr collapsed (while in earlier times the enemies of Sauron had been unable to completely destroy Barad-dûr because Sauron was still alive) and Mount Doom had a massive eruption after which it became dormant forever, similar to how Darkar's fortress and all the caves around it collapsed after Darkar was defeated.
- In The Lord of the Rings, the darkness at the heart of Mount Doom was so intense that even the light of the Phial of Galadriel, containing the light of Eärendil's star, the purest light of the universe of the Lord of the Rings, was subdued and Frodo's will was broken and he ended by becoming corrupted by the One Ring. This is similar to how the evil at the heart of Darkar's realm was so powerful that even Bloom, who possessed the Dragon Flame, the most powerful power of light in the Magic Dimension, was severely weakened.
- The realm of Darkar may also have been partly inspired by Mordor, the land ruled by Sauron. Mordor is a land peopled by evil creatures such as orcs, trolls and giant spiders and where normal life cannot exist.
- Utumno, the fortress of Morgoth, the original Dark Lord and Sauron's master in The Silmarillion, might also have been an inspiration for Darkar's fortress as both are very deep underground fortresses made of many caves, tunnels, corners and rooms.