Dark Realm by Mary Vigliante Szydlowski

Satan has given the people a savior. A Messiah! While he rules the netherworld, his daughter, Ara, oversees his worldly realm. She's the keeper of the faith. Her frenzied followers worship her with the same zeal they do the King of Demons. Her concert tours and sermons overflow with true believers, willing to burn themselves alive to show their devotion. The clergy and aristocracy loathe her. They think she's a fanatic who riles up slaves and lowborns with her foolish talk of salvation. In a place where all Satan's children bear his mark, coal-black eyes, her blue eyes are seen as an abomination by the clerics. Though they hate her, they're afraid to rise up against her…until the rumors start. Something's wrong with Ara. She's acting irrationally! Hearing voices! Seeing things that aren't there! She's got to be stopped before her delusions and rantings upend the world!