Last of the Nephilim / Bryan Davis.

Enhanced descriptions from Syndetics:

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 who originated in the depths of Hades; 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.