User blog comment:27finnsprincess/Finn may be the Tragedy/@comment-5125870-20121023002731/@comment-5465727-20121023004704

Well, Finn had a premonition dream, which according to this Wikia is pretty much the same thing as a croak dream. Given from the context perhaps a premonition dream is which one were you see some else dead, being both Finn and Jake feared Billy being dead. So perhaps Billy is dead, or close to being dead because he's possessed by the Lich.