The High King's Tomb Book Three of Green Rider

For Karigan G'ladheon, the call of magic in her blood is too strong to resist. Karigan returns to the Green Riders, the magical messengers of the king, to find she's badly needed. Rider magic has become unstable, many Riders have been lost, and the Rider corps is seriously threatened. The timing couldn't be worse. An ancient evil, long dormant, has reawakened, and the world is in peril. Karigan must face deadly danger and complex magic to save the kingdom from certain doom.