I am unfamiliar with Der Freischutz. A wiki read and I can see that it's an opera where a devil figure makes magic bullets in exchange for the gunner's soul. Something like that? While I am reluctant to include feranmuts that are non-canon, I will withhold judgment until I see what you've come up with. I used to think that the only feranmuts were the Sui siblings, but Il Siracusano has shown me otherwise.