Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Once upon a time, an angel and a devil feel in love. It did not end well.
The epigraph for Daughter of Smoke and Bone is original, not from another source, as most epigraphs are. There are a few of these little "epigraphs" sprinkled throughout the novel, usually when a new plot arc begins or the narrative P.O.V. shifts to another character.
Each one begins with "once upon a time," which adds to the general fairy-tale quality of the novel. Plus, they provide you with little hints as to what's going to happen—so they're really a foreshadowing device.
Unlike most fairy tales, though, you'll notice that this story doesn't end with a happily ever after. Do you think Karou's story ever will have that perfect ending?