According to legend, Illyius was a shy, penniless orphan who lived a very long time ago in a wizarding village in the mountains. The hamlet was surrounded by a thick forest, where a Dark wizard, Raczidian, resided in a black castle guarded by Dementors. For many years, a sort of truce existed between Raczidian and the people of the village, with Raczidian leaving the villagers alone, and the villagers staying out of the part of the forest where his fortress was located.
However, realising the danger that Raczidian's Dementors posed, the villagers duly taught the Patronus Charm to every generation. While not everyone was able to master the spell, there were always just enough Patronuses to ensure that, in the event Raczidian ordered an attack, the village would be protected.
Thus, when Illyius reached the age of 17, the village elders showed him and the village's other young witches and wizards how to cast the Patronus Charm. Illyius was able to produce a full corporeal Patronus, but to his humiliation, it took the form of a mouse. Having never seen such a puny, feeble-looking Patronus before, everyone laughed at Illyius, and the village elders instructed him never to use the spell again.
A short time thereafter, Raczidian took notice of Eliana, a beautiful young girl who lived in the village (who Illyius had never spoken to due to his shyness), while she was out picking berries in the forest. Deciding that she would make an ideal wife, Raczidian sent a demand to her parents, but they refused to consent to the marriage. Raczidian then threatened to send his Dementors to destroy the village unless they delivered Eliana to him.
The village elders had a meeting and agreed to put up a fight. Eliana was sent to Illyius's tiny shack, and Illyius was ordered to sit out the battle and keep her company, as his Patronus was thought to be too weak to be of any use.
At first, the villagers managed to hold back the torrent of Dementors that attacked the village with their Patronuses (which included bears, wolves, and wild boars), but gradually the sheer number of foes began to overwhelm them. The Patronuses grew faint, and the spellcasters collapsed, or fled for their lives.
Eliana begged Illyius to do something. So he cast the Patronus Charm, and despite its diminutive size, the mouse shone with a brilliant light, bringing the Dementors to a halt as it nimbly moved through the ranks of fleeing villagers. Enraged, Raczidian decided to enter the fray himself, and tried to summon a Patronus to ward off Illyius's mouse. However, he failed to remember that only the pure of heart can produce a Patronus, and thus for the first time in history, it was revealed what happens when a competent but unworthy wizard or witch attempts the spell. Maggots shot out of Raczidian's wand and quickly devoured him as they engulfed his entire body.
The villagers hailed Illyius as a hero. He married Eliana and had a long happy life and from that day the most esteemed Patronus in the village was the swift nimble mouse.
Personality and traits
Illyius was noted for his extreme shyness. He often found himself tongue-tied and unable to speak to others. However, he also seems to have been resourceful, as he was able to survive on his own without his parents. He also had advanced magical ability, as he was able to produce a full corporeal Patronus when others could not.