This is absolutely my favorite song from Phantom of the Opera.
Initially made famous by Michael Crawford, the actor who originated the role of the Phantom in London's West End and on Broadway, the popular song has sold millions of copies worldwide, and has been translated into may different languages.
In the musical, it is sung after the Phantom lures Christine Daaé down to his lair beneath the Opera House. He seduces Christine with "his music" of the night, his voice putting her into a type of trance. He sings of his unspoken love for her and urges her to forget the world and life she knew before. The Phantom leads Christine around his lair, eventually pulling back a curtain to reveal a mannequin dressed in a wedding gown resembling Christine. When she approaches it, it suddenly moves, causing her to faint. The Phantom then carries Christine to a bed, where he lays her down and goes on to write his music.