Yllidith's History
Pre-Series
During the height of the Earth Fairies' reign over Earth and some time after the development of the Black Circle, Yllidith kickstarted the Great Fairy Hunt. With his immense power and mastery over casting illusions in particular, many wizards who also wished to overthrow and eradicate the Earth Fairies began to follow his lead. He was both highly feared and respected by all who followed him, as they even believed themselves to be invincible under his leadership. Some of his followers, namely Ogron, even wished to usurp Yllidith for his title, but Yllidith's power proved to be too strong for any who dared.
At some point, Yllidith decided to attack the heart of the Earth Fairies' kingdom directly by leading his legion of followers to Tir Na Nog to try and completely wipe out their enemies and take their castle. During this event that would be later known as the Siege of Tir Na Nog, Yllidith was confronted by Nebula, but was easily able to overpower and defeat her due to the vast difference in their powers and experience since Nebula was still too inexperienced to even match his illusions.
It was not until Morgana, Earth Fairy High Queen, and her three Major Fairies joined their powers together that Yllidith was finally defeated and, without his leadership, the Fairy Hunters' efforts to take Tir Na Nog quickly went up in smoke, as they were driven off the island. Yllidith would later be imprisoned within the dungeons of the castle he wished to take for himself, locked within a special cell fortified by Morgana's magic behind a powerful seal he could not break. To ensure that he never made any attempts to break free, Yllidith would also be monitored under the watchful eye of Nebula, the very fairy he had first defeated, as she rose to the rank of Major Fairy herself. Additionally, with Yllidith no longer capable of leading the Fairy Hunters, Ogron finally found the chance he was waiting for and took it without hesitation by taking his place as leader.
Yllidith remained trapped within the castle dungeons for centuries, even as all but one Earth Fairy was strapped of her wings and imprisoned, and later freed as the remnants of the Fairy Hunters were wiped out themselves. Eventually, as the castle was repurposed into a school, two young fairies had wandered into the dungeons completely lost. Seeing his golden opportunity for escape, Yllidith made contact with the two fairies through telepathy and tricked them into believing that he was nothing but a strange professor who happened to live under the school. He then began to teach these two who to use and cast various magical illusions so long as they promised to keep their lessons a secret, and had them channel their magic into the seal keeping him imprisoned with every lesson until it cracked enough for Yllidith to set himself free while Nebula was not monitoring him.
Season 6
Yllidith is first mentioned in The First Fairy Hunter after the Winx witness Nebula reacting negatively towards two of her students, Lydia and Nadine, casting illusory spells, mentioning a ban over all illusion magic over Tir Na Nog College. Once Morgana recounts the tale of the Siege of Tir Na Nog to the Winx, Bloom, Stella, Tecna and Musa go about looking for the two recently-suspended fairies, now suspicious of where they came to learn of such spells.
When they are unable to locate them, the four fairies alongside Morgana journey down to the school's basement and find Nadine and Lydia sneaking around, trying to get to a mysterious professor who wishes to keep their lessons a secret. Now caught, both fairies recount their encounter with this professor and show the pillar he had them direct their magic onto. Once Morgana recognizes the cracked M on the pillar to be the very seal she created to keep Yllidith imprisoned, Yllidith emerges from behind her to strike her down with a spray of fire and finds himself surprised with how easy it was for him to knock the former Earth Fairy High Queen out of the fight. He is then confronted by the four Winx fairies as they transform, but Yllidith simply surrounds them in a small army of his magical duplicates before teleporting out of the basement entirely.
With the Winx occupied with his doubles, Yllidith sets his sights on Nebula next and goes about luring her out by transforming into Morgana as he makes his way to her office. When he makes it to his destination, Yllidith reprimands Nebula for being weak-willed and unfit to stand as the current high queen but, just as she is about to leave the school like he had planned, three of the four Winx fairies that he left in the basement catch up to him and out his disguise.
Yllidith launches a spray of dark fire at the light fairy, but his momentary rage subsides as he remains confident with how none of them will be able to defeat him even if they know where he is. He then goes on to mock his enemies when one of their spells fails to capture him, while Nebula keeps getting frozen in her fear of him and his spells.
Eventually, Yllidith casts a large enough illusion to make the College appear as though it is crumbling to everyone inside, and destroys the electronic device of one of the Winx to keep them from seeing through any more of his spells. Unfortunately, Nebula wills up enough courage to completely dispel of his school-wide illusion and, before he can retaliate, Yllidith suddenly finds himself surrounded by Morgana and the other three Major Fairies responsible for the defeat he faced centuries ago. To make matters worse, he is unable to see that the four fairies who suddenly appeared where nothing more than illusions created by the very two fairies he taught those spells to quickly enough, as his being momentarily distracted causes him to be bound up in magical vines and blasted into oblivion by a powerful joint-spell unleashed by the Winx and Nebula.
Yllidith's permanent defeat also allows for Nebula to lift the illusion magic ban over the school as she now encourages all her students to cast any spells they wish.
