Karen Anderson & Poul Anderson 
Dog and the Wolf 
King of Ys Book 4

Ys has fallen, murdered by her god in an act of senile vengeance. Now Gratillonius, once King of Ys, must strive to save a remnant of his people from utter destruction – then use them to rescue civilisation itself, as the light that once was Rome flickers out and barbarian night descends upon the world.In the process, he will give rise to a legend that will ring down the corridors of time . . .