Moribito Series - Uehashi, Nahoko

At a time when the balance of nature still held the civilizations of mankind in thrall, young prince Chagum has been accused of being possessed by an evil spirit: the water demon, which once perished at the hands of the emperor 100 years ago. This could possibly mean great misfortune. Not only for the Imperial family, but for the entire world as well.

Court advisors only see one solution. Chagum must be put to death by his own father's hand. His salvation is in the form of Balsa, a spear woman and mercenary from Kanbal, the kingdom across the mountains. Her skills are legendary, and although reluctant, she is held by a mysterious vow to save eight souls before she dies. Can she fend off an entire empire and make Chagum her eighth soul?

(Source: RightStuf)

The first two novels of the Moribito Series were published in English by Scholastic from April 1, 2009 to May 1, 2009.