The mood of The Phantom of the Opera is one of intrigue and impending doom. In Chapter 1, the author creates a sense of intrigue by describing secondhand accounts of the Phantom:
Was there any truth in all this? [...] He had found himself face to face – in a manner of speaking, for it could hardly be called a face – with the mysterious figure on the little staircase by the footlights that leads directly down to the mezzanine floors below the stage. He had only caught a glimpse of it – for the ghost had fled – but that sight had made an indelible impression upon him.
Here, the chief stage machinist Joseph Buquet describes his glimpse of the "ghost." Rhetorical questions about truth and multiple conflicting accounts of the Phantom create a mysterious mood that intrigues the reader.
The Phantom does many sinister things, and despite a general uncertainty about his presence, the reader anticipates a pivotal moment of violence. From the initial descriptions of Erik's face, to the moment the chandelier crashes down during an opera, to Erik's ultimatum to Christine, each consecutive scene darkens the mood and creates a sense of dread about the Phantom's next violent action. In a sense, the reader's dread mirrors that of the opera-goers; although Leroux provides clues about the plot, the reader still might be shocked to learn of the extent of the Phantom's cruelty. The novel's dark and dreadful mood supports the events in the story.