The Ruby Key (Moon and Sun)

Mankind is Sunkind
And rules by the light;
Nightlings are Moonkind,
And rule in the night;
Or there will be war.

Human 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: Genna and Dan's 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.