As a scourge of grisly murders sweeps the globe, archaeologist Erin Granger must again join forces with Army Sergeant Jordan Stone and Father Rhun Korza to search for a treasure lost for millennia. But the prize has already fallen into the hands of their enemy, a demon named Legion, before whom even the walls of the Vatican will fall.

The search for the key to salvation will take Erin and the others across centuries and around the world, from the dusty shelves of the Vatican's secret archives to lost medieval laboratories. All the while, they are hunted by creatures of uncanny skill and talent.

Soon Erin will discover that the only hope for victory lies in an impossible act - one that will destroy not only her, but all she loves. To protect the world, Erin must walk through the very gates of Hell and face the darkest of enemies: Lucifer himself.