Who is Erik?
The Phantom of the Opera, or Erik, as has now been revealed, is now known to be a simple man. Christine describes his face as similar to that of a decayed corpse, coated with rotting flesh in the place of smooth skin. Erik himself told Christine that his mother gave him a mask when he was young, possibly implying that he was born disfigured.
It is also revealed that Erik lives in the walls of the Opera House. Somehow, the building was constructed with a set of corridors that he can use to traverse the building, much like that of a phantom. It is also revealed that there is ample space within this system of corridors for a horse (who can apparently walk up stairs, but that is neither here nor there).
Finally; Erik is a killer. Whatever his mental state, he was so obsessed with Christine and the power that came with being the Phantom of the Opera that he killed and maimed. His mind has grown darker than the shadows he stalks.
Monstrous Poetry 