Penelopes weapon (?):

In the libary of her aunt she found a book, filled with demonic power...and notes. Well, she was never really musical, but ambitious, so she took Vincents violin and with his help she learned how to play the notes.
But every mistake left a scar on her skin and the demons, who were no longer trapped (still connected though) in the book, visited her in her dreams.

With the book Penelope can controll demons, call rats and many other fun things, which cause nightmares..yay