The Phantom of the Opera was a character in a musical and novel of the same name.
This titular individual was described by The Doctor as being "a tormented character who was soothed by music." In 2377, he named a member of an unknown species "Fantome" after this character, as music appeared to have a similar effect on him. (VOY: "The Void")
The Phantom of the Opera was originally a novel written by Gaston Leroux, which was adapted into several dramatic, film, and musical adaptations. It told about a beautiful opera singer who is kidnapped by a mysterious, tortured phantom that haunts the theater. (Star Trek Encyclopedia, 4th ed., vol. 2, p. 135)