Illyius was a shy wizard who saved his village from Dementors by using his mouse Patronus.
Illyius lived near the castle of the Dark Wizard Raczidian, who was guarded by Dementors and kept to himself. But one day he decided to marry after seeing a young girl named Eliana,. When her parents refused to let him wed their daughter, the outraged Raczidian ordered his dark creatures to attack the villagers, who tried to hold them off by using their patronuses, including bears, wolves, and at least one wild boar (BoS) .
Illyius was guarding Eliana and she begged him to save the village, but his Patronus was a mouse, which was considered weaker than other large animals. But when he produced it against the Dementors of Raczidian, the mouse "shone like a star" and saved the day (BoS).
Raczidian was furious that his creatures were stopped by a mouse Patronus, and he decided to cast one of his own. But because he was so Dark himself, the "Expecto Patronum" spell created horrible maggots that crawled out of his wand and destroyed him (BoS).
Illyus and Eliana married and lived happily ever after.