Conflagration by Mick Farren
"Conflagration" continues the story, begun in "Kindling, of The Four", which alone can challenge the invading might of the Mosul, a ravening horde directed by a fanatical priesthood who can control other world beings and use them as weapons of war. The Kingdom of Albany fights a decisive battle to take back the Virginia Freestate, infantry and cavalry clashing on the ground while The Four wage their own war on the supernatural plane.