The most important component of a Phantom of the Opera costume is the mask. A simple black or white domino mask, which covers the area around the eyes, will do but it is preferable to use a carnival-type mask of any color that covers the face from the forehead to beneath the nose.
The rest of the costume can consist of a tuxedo, or at least a black jacket and pants, white shirt, and black or white bow tie. A cape, as often worn with a vampire costume, adds to the look. A Dracula costume can easily be changed into a Phantom of the Opera costume with the simple addition of a mask.
For added effect, you can use make-up to make the part of your face that will be covered by the mask look hideously ugly. You can then dramatically unmask yourself towards the end of the Halloween party or whip off your mask to frighten trick-or-treaters.
A father and daughter can dress as the Phantom and Operetta, his daughter in the Monster High universe.
Be warned that if you wear a Phantom of the Opera costume, other party-goers might assume that you, like the original phantom, possess a great musical talent, and may call on you to sing a song or play the piano.