From Chris Wickham’s Medieval Europe (190), citing a much longer account of the story in N. Caciola’s Discerning Spirits (87-98). He fails to mention the contemporary source, Richer of Sens:
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“Urban communities also often valued having a recluse or other ascetic figure in the town, as a sign that the town was special. Sibylla of Marsal was an earlier instance, a beguine of exceptional religious commitment in a small town in Lorraine, who fasted and had visions and in 1240 began to attract pilgrims to Marsal; the inhabitants had no problems with this at all, and nor did the bishop of Metz, who had come in person to investigate, once he encountered the demon whom Sibylla was fighting. Only more detailed scrutiny revealed, apparently by chance, that Sibylla was faking it, to the extent that she had made her own demon suit and dressed up in it.”
Caciola’s scene-setting taps the absurdity better than Wickham’s account:
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“…the demon was seen scurrying through the streets and squares of Marsal at night. Whenever this monster encountered local inhabitants, it would introduce itself as a devil and complain loudly about the attacks it endured from “that nasty, impious virgin Sibylla.” On another occasion the demon appeared to a group, including the bishop, in order to complain that Sibylla’s prayerful intercession with God had deprived it of the soul of a recently deceased local sinner. Her intervention, the demon continued, would cost him dearly when his master Satan learned of it.”
Extracts 