Once upon a time, twin girls were born to the Rulers of Heaven. The Rulers named their daughters Sun and Moon, gave them a magical Orb, and sent them to Earth to live and learn among mortals. Every nine years the twins were to pass the Orb from one to the other, so that their heavenly powers might remain balanced. For hundreds of years they did as their parents had instructed, ruling in harmony. Sun, the older twin, was much engaged with the world and experiences of the flesh- even taking mortal lovers and eventually giving birth to a daughter she named Starlight. Moon, however, was more intrigued by the realms of magic and the mind, and thus held herself apart from most mortals. Eventually there came a day when Moon was working on a particularly intricate and difficult spell, and did not want to interrupt its progress by returning the Orb to Sun at the appointed time. Moon knew Sun never did as much with the Orb as she herself did, and felt her sister wouldn't mind waiting a while longer for its return. In point of fact, however, Sun was enraged by this breach; but so long as Moon held the Orb, Sun was less powerful than her twin, and could do nothing in retaliation.
During this period of estrangement, a demon stole Starlight. Sun was able to rescue her daughter, but discovered to her dismay that the demon had separated the girl's spirit from her body, and had hidden the spirit away before vanishing back to Hell. In despair, Sun transformed Starlight into a beast, so that her body would remain safe while Sun attempted to locate and rescue Starlight's spirit.
By this point Moon was deeply regretting her selfishness: she knew that if Sun had been in possession of the Orb, as she should have been, the demon would never have been able to get close to Starlight. And so Moon started looking for a way to recover her niece's spirit. As she did so, she sent one of her servants to return the Orb to Sun.
This did not go as planned.