The Ruby Key — Holly Lisle

IN THE TWILIGHT REALMS OF THE NIGHT WORLDS, A DARK MAGIC IS ON THE RISE....
When Genna and her brother Dan learn that their village chieftain has made a dangerous deal with Letrin, ruler of the nightlings, offering the lives of his people in exchange for his own immortality, they strike their own bargain with the nightling lord. But the stakes are even higher. Now, the siblings must embark upon a journey along the Moonroads, and bring back the key to Letrin's downfall. . . .