Last of the Nephilim by Bryan Davis

Book 3 of Oracles of Fire

Summary:

A single lie is spoken, allowing the Nephilim, a race of giants from ancient times, to invade Second Eden, a peaceful civilization in an alternate dimension. While Earth and Hades are locked in a catastrophic merging of their two realms, the prophet Enoch works to gather an army of humans and dragons from each world to go to war against the Nephilim. Included in that force are a small girl with prophetic power; Billy Bannister, the heir of Arthur and hero of the Dragons in Our Midst series; and Bonnie Silver, his fellow warrior.

The two Oracles of Fire, Sapphira and Acacia, are called upon to protect the forces of good, including Bonnie, who is pursued once again by the most powerful of all dragon slayers, Sir Devin, the dragons' bane. 

With giants battling dragons, and a rogue dragon taking the side of the forces of evil, a cataclysmic battle erupts, endangering every life in the land of Second Eden. Another of the Nephilim, the most powerful of all, rises from the dead to join the battle. But whose side will he be on?

Abraham, the leader of Second Eden, has a choice, either execute a grieving widow, the woman responsible for the entry of the evil giants, or face the greatest of sacrifices by which he can only delay the inevitable, the ultimate war that could lead the evil forces to the gates of Heaven itself. 

Opening Lines: 

"Mardon, I know you don't enjoy being dead, but you must come to grips with reality."

From her seat on a flat rock, the lithe woman leaned over a stone table, the only furniture amid a collection of oddly shaped boulders draped by a black sky. Her movements troubled the red flame of a pewter-gray candle embedded in a hardened splash of wax on the table's surface. 

Laying her hands over the intertwined fingers of the man sitting opposite her, she spoke with a breathy and ominous voice. "If you try to follow your giants to Second Eden, you will be swept into the lake of Fire where you will burn with your father forever. Dead souls such as yourself cannot survive outside of Hades."