Just read some posts above. As already mentioned, it's the Faust opera and it has a very special meaning
to quote the wikipedia artikel about Faust:
The story concerns the fate of
Faust Nodame in
his her quest for the true essence of
life music. Frustrated with learning and the limits to
his her knowledge and power, (s)he attracts the attention of the Devil (represented by
Mephistopheles Milch eh Streseman), who agrees to serve
Faust Nodame until the moment (s)he attains the zenith of human happiness, at which point
Mephistopheles Lord Sith Streseman may take
his her soul.
Faust Nodame is pleased with the deal, as (s)he believes the moment will never come.
Though last sentence may not be true.
Edit:
THIS beautiful theme would also be a fitting as BGM, however it may be a "bit" out of place.
Just tried it and wow, works perfectly. Let the music fade in as he activates the magic circle.