The Unconquered Mage — Melissa McShane

Once a thief and secret mage, then a leader of mage warriors, Sesskia is now Empress-Consort of the combined countries of Balaen and Castavir. But that title is far from secure.
Sesskia and her husband Cederic have only two-thirds of the Balaenic Army to face off against the former God-Empress of Castavir. None of the Lords Governor of Balaen or the consuls of the Castaviran Empire support them. And the threat of assassination is all too real.
Worse, however, is the realization that something is happening to magic. While the convergence brought the two worlds together, it failed to unite their different magics -- and no one knows why.
With the clash between the new Empire and the God-Empress looming ever closer, Sesskia and the mages must find a way to complete the convergence, even if it costs them their lives.