The ruby key

Humans and nightlings are never to meet, but when Genna and her brother, Dan, venture into the old forest at night, they encounter a nightling slave who reveals a terrifying secret: their village chieftain made a dangerous deal with Letrin, ruler of the nightlings, offering the lives of his people in exchange for his own immortality. To save their people, their family and themselves, Genna and Dan 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...