The Queen's Blade IV, Sacred Knight of the Veil

The Queen’s Blade vanished fifteen years ago, and plotting Cotti Princes threaten the three kingdoms’ peace again. Only the invisible assassin can save the child Queen, but first he must become a Sacred Knight of the Veil, an office he scorns. However, he’s sent into the desert after making a bargain with Shamsara… on a seemingly impossible mission that, if he succeeds, will earn him an unequalled reward.