Shadow Queen (Crowns #2) by Nicola Tyche:
 
For ten years, her kingdom has warred against the Shadow King. Now, she’s his queen.

Treachery Within …
Reaping the consequences from her decision to ally with her kingdom’s greatest enemy to avoid war, Norah Andell, queen of Mercia and salara of Kharav, faces opposition from both realms and struggles to hang on to the fragile peace between them. Treachery lurks in the shadows, intent on subverting everything she has worked to build, and threatens to plunge the kingdoms back into a deadly war.

A King Divided …
The Shadowlands are a warrior kingdom, and Mikael Ratha Shal their ruthless king. Battle and blood have made them strong. Kingdoms fall to them; men fear them. But his new queen strives for peace and compassion, and while he has her hand, he wants her heart—even though her heart still holds love for another. Norah tests the limits of his mercy, where he risks not only the allegiance of his kingdom, but also his crown.

A Rising Threat …
With the joining of Mercia and the Shadowlands, the loyalty of old allies crumbles. All the while an unexpected threat is rising. Leading a rebellion in the nearby kingdom of Rael, a usurper seizes the throne and is crowned king. As thousands flock to his banner and his righteous cause, he turns his vengeful eyes on Mikael and the Shadowlands. Norah must choose to heed the demands of her own kingdom and return home to safety, or risk their dissent and stand with the enemy she now calls her husband.