The Mirror of Cagliostro (Fantasy and Horror Classics)

The young Bostonian Harry Langham travels to Paris while researching his thesis on Cagliostro, who was reputedly the most powerful black magician of the eighteenth century, although equally likely to have just been a clever charlatan.
Professor Thibaut, a leading authority on Cagliostro, warns him against following up on his information as "evil never dies", but Langham isn't to be dissuaded.