Shinai naru Phantom e: Opera-za no Kaijin

There lies a phantom in the opera house of Paris. The being named "F" is rumored to not only be a talented magician and composer, but he is also blessed with a hypnotic singing voice. Due to his odd requests, no one dares to go against his wishes.

Christine Daaé is a part of the chorus at the opera house. As the daughter of a violin player, she has been surrounded by music since she was a child. While trying to catch the only memento of her father from falling into the river, Christine is saved by a strange man with red eyes, who disappears as quickly as he appeared.

When the prima donna Carlotta quits from an upcoming show, Christine becomes her replacement but has no confidence in singing by herself. In hopes of meeting F, Christine goes to the central-western balcony, and luckily, he appears before her. She is shocked to find out that the elusive ghost is the same man who helped her not too long ago. He reveals his real name is Erik and tells Christine he will become her teacher on one condition: she must promise she will not love anything other than music—not even Erik himself.

(Source: MAL Rewrite)