"Forgetting that only the pure of heart can produce a Patronus, he tried to cast a guardian that would shield him from Illyius’s mouse. Only now was it discovered, for the first time, what happens when an unworthy but skillful wizard attempts the Patronus Charm. Maggots gushed from the end of Raczidian’s wand. They crawled all over him, hiding him from sight, and before the villagers’ horrified eyes, he was devoured."
-- Miranda Goshawk (BoS)
Raczidian was a Dark wizard who lived in a castle guarded by Dementors (BoS).
In a nearby village lived a girl named Eliana and Raczidian chose her for his bride, but her parents refused. In anger, Raczidian sent the Dementors to attack the village, and the wizards who lived there put up a fight with their own Patronuses, including bears, wolves and a wild boar. But the Dementors were too strong, so Eliana pleaded with the wizard who guarded her, a shy lad named Illyius whose Patronus was merely a mouse and thought to be weak. However, when he produced it, his Patronus “shone like a star” and held back the Dementors (BoS).
Raczidian was even more furious now because the mouse was ridiculously small yet had stopped his dark guardians. He decided to cast a Patronus of his own to fight against the glowing mouse, but it was a terrible mistake. As a Dark wizard Raczidian was unworthy to create a spirit guardian. As he said “Expecto Patronum,” maggots poured out of his wand and covered him, devouring him in front of the whole village (BoS).
Illyus and Eliana married and lived happily ever after. And no one in the village ever made fun of a mouse Patronus again.
The story of Raczidian seems to be a cautionary tale from Miranda Goshawk about impure wizards attempting to produce a Patronus. And yet, the borderline sociopath Dolores Umbridge was able to create a glowing cat to protect herself from Dementors at the Ministry of Magic. Perhaps the best explanation for why Umbridge could create a Patronus lies in her attitude, that a "true and confident belief in the rightness of one’s actions can supply the necessary happiness" (Pm-Patronus Charm)." In other words, although Dolores was a Dark witch with a horrible life, she believed in herself and that made her happy enough to master the Patronus Charm.