Female monasticism and the conversion of the Jews were both major concerns for the ecclesiastical establishment, as well as for Italian ruling elites, after the Council of Trent (1545–1563). Hence, the monachization of baptized Jewish girls acquired a unique symbolic significance. Moreover, during this period cases of demonic possession were on the rise, and so were witchcraft accusations. This article explores a case from late sixteenth-century Mantua in which Jewish conversion, female monachization, demonic possession and witch-hunting all came into play in a violent drama. Drawing on unpublished documents as well as on chronicles and hagiographies, the article elucidates the mental toll that conversion and monachization took on the Jewess Luina, who later became known as Sister Margherita. It delineates her life, which culminated with her diagnosis as a demoniac, and analyzes the significance that this etiology held for the energumen—whose affliction was attributed to her ongoing contacts with Jews—and for Mantua's Jews. The article argues that the anxiety provoked by suspicions that a formerly Jewish nun reverted to Judaism was so profound, that it led to the burning at the stake of Judith Franchetta, the only Jew ever to be executed as a witch in the Italian peninsula.