Yesterday an Overseer passed in the street near me, playing the strange music from the Abbey. Beneath his mask, I could not see his face as he turned the crank on his music box, but I could hear his voice muttering verses from the sacred texts.
Fear overcame me, because it is forbidden to carry bone charms and similar such items for luck. The Overseers are completely unforgiving in this matter. Gripping mine in my pocket, feeling its warmth, I walked away with my head down.
He was heading toward the Royal Physician’s home, that shaggy Sokolov, and something odd occurred. I could swear the charm in my hand reacted to the Overseer’s music, growing cold. Some inner vibration seemed to go still.
I found a book that is perhaps related to this and will study it so that I might understand.
It can be found in the apartment on the south end of Kaldwin's Bridge (which Corvo must enter to advance to the next area of the map) during the mission, The Royal Physician. It is found beside a copy of The Ancient Music and near a rune.