Answer the following question: Generate a question that has the following answer: the Alchemist. for the following movie plot: In 19th century Paris, the famed detective Vidocq (Gerard Depardieu) disappears while pursuing a murderer called the Alchemist. Etienne Boisset (Guillaume Canet), a young biographer of Vidocq's, follows in his footsteps and progressively uncovers Vidocq's investigations. He learns how Vidocq was first called in to investigate a series of assassinations by lightning, which led him to pursue the Alchemist. Etienne discovers that the Alchemist is a wizard of sorts who wears a mirrored mask and kills virgins to maintain eternal youth. Even the sex of the Alchemist is a mystery, because it sometimes utters mocking feminine sighs during combat with Vidocq. Etienne probes deeper into Vidocq's investigations and eventually comes to learn that the Alchemist uses virgin blood to make magical mirrors that consume human souls. At last, Etienne comes to the end of his trail and discovers Vidocq alive and in hiding. Etienne dons a mirrored mask and reveals himself to be the Alchemist. Vidocq has known all along, however, and is ready to fight his nemesis. During their final confrontation, Vidocq defeats the Alchemist, but the end of the film suggests that the Alchemist survives.
Answer:
What is the name of the murderer that Vidocq is pursuing?