It was Erik with an Original Character. A ballerina that was a bit more mature (but not old), I think he was watching her dancing by herself in the middle of the night a few times. Later in the fic he had the Opera House put on the ballet Giselle and he cast her as Giselle's mother. It was a very long story. Rated for mature, lots of sexy stuff I think. I'd love to find it and read it again.