Gallicenae

In book two of the King of Ys series, Gratillonius adjusts to his new role as sovereign of Ys as threats from all sides begin closing in

As the Roman Empire loses its grip on its far-flung territories, the mystical kingdom of Ys in western Gaul is in great danger of slipping into oblivion. Suffocated for years by the rule of a tyrant king, Ys's last hope arrived in the form of a Roman emissary, Gratillonius, who defeated the sitting king to take the throne himself. Now Gratillonius must grapple with the kingdom's political strife and religious tensions while balancing his responsibilities to the Gallicenae, nine wide-ranging witches who have become his wives. Though Rome seeks to spread Christianity, and Gratillonius stands firm in his worship of Mithras, the Ysans hold to an entirely different religion in service of pagan deities who must be obeyed lest grave consequences descend on the fragile kingdom.
 
Gallicenae is the second book in Poul and Karen Anderson's King of Ys series, which continues with Dahut.