Ars Magica: Druesberg
Fulbert of Béziers
Diabolist who believed he was a wizard
A diabolist whom Aqrab deceived into believing he was a Hermetic magus. The party encountered him in Béziers, where he was making use of the Infernal aura created in the city by the massacre in 1209. He considered himself to be a Cathar, although his actual beliefs were infernally corrupted, and he used his infernal powers to resist the anti-Cather crusade.
Fulbert wrote an apocalyptic ‘revelation’, which the magi added to the covenant library as a source on Infernal Lore.
The magi defeated Fulbert and too him back to the covenant as a prisoner. When he learned he had never been a member of the Order of Hermes, he renounced his infernal powers.
(GM note: I can’t actually remember what happened to Fulbert. Is he still your prisoner or did you let him go?)