Whenever you see a pretty (usually blue or black) butterfly show up, things are about to get Symbolic. For millenia, the world has latched on to the image of the butterfly: its metamorphosis from a caterpillar to a butterfly used as a metaphor for death and rebirth. In brighter series, it means "Don't worry, be at peace, the great circle of life continues on." In others, it means "You're going to die and turn into something else and it being pleasant isn't necessarily an option." Dead butterflies are an especially ill omen.