Karen Anderson & Poul Anderson 
Dahut 
King of Ys Book 3

The conflict between the King of Ys and the gods growing, with increasingly devastating results. It is becoming impossible to balance the demands of his own god, Mithras, with those of the gods of Ys, as they demand that he marry his daughter, which is forbidden by his religion. And while the King struggles to preserve his kingdom, others are plotting his demise… Meanwhile, the Roman Empire is occupied by its own internal struggles, and the barbarians sense an opportunity to attack. Filled with rich historical detail and a gripping fantastical narrative, this wonderful mixture of history, legend and fantasy continues the compelling story begun in Roma Mater and continued in The Gallicenae.