Blighted Fortress - David E Dresner - Bog

This story begins in modern Chicago then moves to fifth-century Transylvania.
The teenage protagonists, Glenda and Traveler, are sent by Theo, their sanctuary god, deep into the primeval forests of the Carpathian Mountains. They must locate then neutralize an ancient demi-god called a "fire beast".
The two must also survive the daunting challenges of isolated fifth-century Transylvania. Rome is now in its final death throes and the hoof beats of Attila the Hun echo across Europe. Civilization and Pax Romana are distant memories.
Magic, wits, and youthful courage must combine to face the fire creature. Survival, much less winning, is up for grabs.