Defeating Sir Nautilus with the Safekeeper's ring from Spirit Reforged will cause him to "die" instantly without his reconstruction animation and you get three treasure bags as if you defeated him three times at once. It will also prevent progressing in his dialogue, since it won't register that you completed the "challenge". Edit: maybe I should also report it to the developers of spirit reforged as well. It is not anything particularly game breaking, just an interesting observation.