Glowing Eyes
One of the oddities about Erik, and a reason that many of the characters think he is not entirely human, is his eyes. They glow in the dark like a cat's, and seem somewhat luminous. It is perhaps this trait that allows Erik to see so well in the dark and to move so freely underground.
From time to time, Christine and Raoul see a pair of glowing yellow or orange eyes. Frequently it means that Erik is spying on them. At one point, Raoul shoots a pistol toward the glowing eyes and manages to wound what everyone thinks is a cat.
Monster Imagery
As the tale progresses, Erik is described by his friend the Persian less as a human being and more as a "monster." His past history as a torturer and killer, and the sadistic way in which he frightens, stalks, and sometimes murders people, are factors in the Persian's decision to apply that label. Admittedly Erik has a very unattractive face and skin, and suffers from bad body odor such that he frequently resembles a corpse. Yet the animalistic way in which he moves silently, sees in the dark, and does not feel himself constrained by human morality are all reasons why the Persian, Christine, and Raoul do not see Erik as a human being.
In Erik's mind he is being rejected because of his appearance. In reality, people are terrified of the unpredictable and violent things he does.
Animals
Some of Erik's traps are decorated with animal images. In Erik's torture chamber Raoul and the Persian are treated to scenes of predatory felines. The grasshopper and the scorpion figures, one of which is attached to a gunpowder charge, are present in one of Erik's sadistic machines.
Strangulation
One of the tricks Erik plays on Carlotta takes place when he causes her to lose her voice, in retaliation for a similar trick played on Christine. Attacking the neck and throat is one of Erik's favorite things to do.
The Punjab lasso, which appears to be a narrow thread-like lasso used by Erik for strangling his victims, is present on the floor of the torture-chamber. The Persian remembers it from the times Erik used to use it to amuse a spoiled and somewhat homicidal young sultana before coming to Paris.
