Waltz for One is a music from the Plague of Shadows soundtrack. This is the dance theme of Mona alone and a slow-paced and moody arrangement of Waltz of the Troupple King. It is Music Sheet #7 in Plague of Shadows. It can also be listened to in Sound Test after completing the campaign once.
Music Sheet #7 is unlocked by giving Percy 10 Scrap Sheets.
"Relaxing song, but so lonely..."