1915. As the second battle of the Isonzo Front rages on the Italian Austro-Hungarian border, war threatens to engulf the Inquisition as dark forces muster amongst the most fanatical servants of the Catholic Church.

Prior to his murder, a desperate priest sends a secret letter to his brother serving in the Italian Army. Now this young soldier, destined for the horrors of a frontline high above the clouds, carries with him a letter which holds the key as to why terrible satanic rituals are being committed and by whom.

Drawn into this conspiracy and hunted by agents of The Darkest Hand, old rivals must put aside their differences to discover the contents of the letter before it's too late. But unity comes at a price for this unholy alliance. While the war rages, old enemies return from the dead and conspiracies weave tighter and deeper still into the heart of the Vatican. Only Poldek Tacit, the most determined and unhinged inquisitor of them all, can hope to push back the forces of evil and unite those for good. But what happens when Tacit finds that the path he walks has already been prophesised and that where it leads threatens the very future of a world already on the edge of the abyss?