Bringer of the Scourge - Song of the Scourgelands, #1

When the end of ages comes for the empire, the princess must rescue herself. Vierrelyne du Talorr, the last living daughter of the tyrant king, waits locked in a tower cell for the prophesied apocalypse only she can prevent. An army of three brittle allegiances, united under a rival prince, aims for the throne and lays siege to the castle in search of the princess and the fabled weaponry of the empire.
With the aid of her mentor in music and swords, and a desperate cultist sent to find her before the mercenaries do, Vierrelyne steals that formidable ancient weapon from her family crypt: a holy suit of armor and a diadem infused with the soul of a demon prince--the Bringer of the Scourge. With it, Vierrelyne discovers an unstoppable power, but the demon within is corrosive, hungry, and dangerously persuasive.
Vierrelyne is haunted by what it means to tame this power bequeathed to her, and by what means she might conquer it. When that rival prince finds her, it will take all the strength she can muster, for, if the prophecy she dreads is true, the very weapons she wields might destroy everything she holds dear.