The Lost Warrior is a tome that can be found on a corpse labelled Lost Warrior in the Battlefields of Eternity in Act V of Diablo III. Its retrieval adds to the People Finder achievement.
Something now inhabits the fortress. I am certain of it. Long have I gazed upon its scarred facade, dreaming of the safe haven its walls offer. But it has changed. The vacant terraces have turned cold. I am so desperate to escape the demons, but whatever is in the fortress may be worse... — Lost Angel