The Skeleton's Knife: Book Three of the Farwalker Trilogy

When 14-year-old Ariel Farwalker sets out to return a dead enemy's knife to its source, she lends strength to an evil that seeks her attention. Aided by a new friend with a smuggler's savvy and a talent for charming the wind, Ariel must travel to the land of the dead to save someone she loves and bury both the knife and the past. If she fails, neither she nor her most beloved friends will survive.