Claudin snaps when he accuses the publisher of plagiarizing his music and strangles him to death. Immediately afterward, the publisher's wife throws etching acid in Claudin's face and he screams in agony.
While there are charges of Lighter and Softer leveled at the musical, there are several sections that are extremely dark and frightening, such as:
The Phantoms first appearance, when his face slowly becomes visible in Christines mirror.
A freakishly realistic doll in the Phantoms lair, who looks identical to Christine and even moves, making Christine faint.
The Phantom laughing wildly as he makes Carlotta croak. He already sounds completely insane - "Behold! She is singing to bring down the chandelier!!!" - and takes malicious, gleeful delight in humiliating the poor woman.
The ballet scene that immediately follows this, as he casts ominous shadows on the backdrop, the dancers get more and more frightened and agitated, and the music more sinister. If you haven't seen the show before, you're just waiting for something horrible to happen; if you have seen it before, you're waiting for Joseph Buquet to be hanged from his Punjab lasso.
Just after the sweet love song "All I Ask Of You" is over and Christine and Raoul depart — the Phantom emerges from behind the statue of the angel, having heard everything. Audiences have been known to gasp in horror, knowing that the man who's already killed tonight has heard everything...
The chandelier first shaking as the Phantom screams in rage, and then swinging down from the rafters into the stage, appearing to nearly hit the characters. Even if you know what's coming it's still a terrifying sight (particularly if you're sitting in the stalls near the stage).
The famous chandelier scene was altered in the Las Vegas production, where it's moved from the end of Act 1 to the end of "Point of No Return" (like the 2004 film). But it doesn't swing into the stage as per tradition; instead, it drops right into the audiencenote Or rather, it drops dangerously close to the audience before the lights black out, but it still made for a pretty intense illusion. Audience members who were lucky enough to be sitting right below the chandelier were in for quite a treat, to say the least (there were even reports of people jumping out of their seats to get out of the way!)