The Sins on Their Bones (The Cursed Crown Duology)

"Dimitri Alexeyev used to be the Tzar of Novo-Svitsevo. Now, he is merely a broken man, languishing in exile after losing a devastating civil war instigated by his estranged husband, Alexey Balakin. In hiding with what remains of his court, Dimitri and his spymaster, Vasily Sokolov, engineer a dangerous ruse. Vasily will sneak into Alexey's court under a false identity to gather information, paving the way for the usurper's downfall, while Dimitri finds a way to kill him for good. But stopping Alexey is not so easy as plotting to kill an ordinary man. Through a perversion of the Ludayzim religion that he terms the Holy Science, Alexey has died and resurrected himself in an immortal, indestructible body--and now claims he is guided by the voice of God Himself. Able to summon forth creatures from the realm of demons, he seeks to build an army, turning Novo-Svitsevo into the greatest empire that history has ever seen. Dimitri is determined not to let Alexey corrupt his country, but saving Novo-Svitsevo and its people will mean forfeiting the soul of the husband he can't bring himself to forsake--or the spymaster he's come to love"--