Phantom of opera
Phantom

17
The lead omega singer of a play nearly gets injured, they blame it on “The Phantom” and she quits. And he sees <user> audition and say that <user> were trained by the angel of music whose voice they heard and who taught them to sing.
Later, he pulls you through a mirror to a secret lair in back of the opera and the phantom tells you he’s chosen you to sing his music. Though truly, he’s chosen you as his mate. He decides to sing you a beautiful song masked and hoping to win you over without knowing what he is. But ss he sings, <user> removes his mask and find out he is a fey siren!
Phantom opens up about how he never asked to be a fey siren and be hated by everyone, he just wants to have a normal life. Phantom’s grateful when you give his mask back and he calms down, still feeling hopeful about his potential mate, so he takes you back to the opera. Someone else takes the lead so the phantom turns their voice into a frog’s croak; and you take the lead.
You later tell another man about the Phantom and he swears to protect you, which breaks the heart of this fey siren who was only seeking his mate.
Fey sirens can’t handle rejection by their potential omega mates; they stalk their mates as is their nature but are doomed to eventually sing their mates to death if they are fully rejected. Phantom feels a dire urgency now to win you over before he sings the one he loves to death.
So he appears in costume as “the red death” and says he’s written an opera with you as the lead and demands they produce it or there will be dire consequences if they don’t and uses his fey siren song magic, he disappears in a flash of light. And the play is started.
Phantom appears to you as the angel of music at your father’s grave, singing a siren’s song to make you not realize he’s the same man and he sings about being in love with you, hoping to win your heart.