When Fortunato comments on how big the catacombs are, Montresor implies that all the bodies in the place are dead members of the Montresor family. There seem to be quite a lot of them. This is before we know Montresor’s name, but it’s implied that he’s a member of that family.
Is our narrator the last surviving member of the family? If so, what happened to the rest of the Montresors? Did he, perhaps, kill them all? Or maybe Montresor is lying and it’s not just Montresor bones in that massive graveyard. Maybe the Montresors were a family of killers, and the catacomb is full of unsuspecting victims, like Fortunato.
Maybe Montresor isn’t really Montresor at all. Maybe he murdered the last of the Montresors and then stole their name, so he could use it for his nefarious purposes. Montresor, if that’s his real name, makes clear in paragraph five that he is not Italian (though where he comes from is never revealed). So if he’s not Italian, what is his whole family doing buried in an Italian catacomb?
Ultimately, we don’t get any concrete information on the mysterious family Montresor; we have more questions than answers. In fact, Poe invites us to such speculations. They enhance the general creepiness of the reading experience, and make us suspect that Montresor, or whoever he is, is an even bigger villain than we might have thought.