In case anyone wasn't sure why the chapter is called "Robert le diable", it's because that's the name of the opera everybody except Haydée is too busy gossiping to pay attention to. It was a very popular opera at the time, but it's also, like all the Byronic references in the Roman opera chapter, symbolically relevant to the Count.
The plot of the opera, which I'm not going to attempt to recount in all its melodramatic convolution here, involves a young man who is tempted to go down a dark path, with his diabolical tempter playing on his love for his fiancée (and particularly his fear of losing her to a Spanish rival) and his grief at the death of his parent.
There are probably a whole bunch of parallels one could draw out, and then argue about whether Dumas was putting that much thought into it, but for the moment I'll content myself with one that Dumas specifically calls attention to: the Count leaves the opera house at the end of Act 3 (out of a total of five) – which is the point at which Robert is freshly committed to his path and has not yet started to recognise that there are going to be Consequences.
who’s up for timetravelling to the 1831 premiere of Robert le Diable and going insane about the now lost og choreography of the ballet of the nuns aka the 1st Romantic ballet:
Ok but in The Count Monte Cristo, do they only go to opera to exchange gossip? Bc it seems like they care more about looking who's in which boxes instead of looking at the stage. The only character genuinely interested in music is Haydee.