Daenyathos

For millennia, the writings of Chaplain Daenyathos have been the guiding force behind the Soul Drinkers, his wisdom taking them from strength to strength. But a dark secret lurks at the heart of Daenaythos's Catechisms Martial, a cancer that will eat away at the Soul Drinkers and ultimately damn them. Spanning the centuries from the Age of Apostasy to his mysterious disappearance, this is the story of the Philosopher-Soldier's life, of his rise... and of his tragic fall.

Read it because
It's a tale that spans the ages and lays the seeds for the events of the Soul Drinkers series... and if you read it before the conclusion to the series in Phalanx, you'll have a much greater understanding of the importance of the Philosopher-Soldier to the Soul Drinkers.