François Prelati was an Italian priest and alchemist who took part in the murders committed by Gilles de Rais in the 15th century. He is thought by some to have been a possible sexual partner of Gilles. He claimed he could summon demons and involved Gilles in this practice.
In fiction
François Prelati has appeared as a character in stories such as Thief of Souls by Ann Benson and Fate/strange fake by Ryōgo Narita.
References
François Prelati Wikipedia(Text) CC BY-SA