You might want to start with the collection of short stories featuring Simon of Gitta written earlier by the same author
CAN A HANDFUL OF HEROES STOP AN APOCALYPSE CENTURIES IN THE MAKING?
Escaped gladiator-slave Simon of Gitta returns to Judea — during the ministry of Jesus of Nazareth — on a mission to avenge the deaths of his parents, seeking revenge in blood against the Roman officials who committed the murders and sold Simon into slavery.
But as Simon travels the Holy Lands with his mentor Dositheus and their apprentices Menander and lione, they gradually become entangled in a complex occult plot designed to call down a monstrous alien entity to herald a new aeon on Earth. The mysterious time traveler John Taggart (from Tierney’s The Winds of Zarr) also becomes involved with Simon as their separate quests converge toward a common goal of saving all life on Earth from extinction.
But can a handful of travelers really thwart a covert scheme backed by the power of the Roman Empire? As the apocalyptic supernatural events slowly unfold, Simon and his allies are in a race against time to prevent the devastation of the world.
Using mystery cults and early Christian Gnosticism as his vehicle, with meticulously researched Roman history and Biblical scholarship, this is author Richard Tierney’s magnum opus: an epic Lovecraftian alternate history dark fantasy novel that features Tierney’s most famous characters, Simon of Gitta and John Taggart. This novel will appeal to fans of historical fantasy and sword & sorcery fiction in the vein of Robert E. Howard, and the elements of cosmic horror and the Cthulhu Mythos will satisfy many fans of H.P. Lovecraft.
