The Demons We See - The Dark Abyss of Our Sins, #1

Society was rocked when the Cathedral appointed Allegra, Contessa of Marsina, to negotiate the delicate peace talks between the rebelling mage slaves and the various states. Not only was she a highborn mage, she was a nonbeliever and a vocal objector against the supposed demonic origins of witchcraft. Demons weren't real, she'd argued, and therefore the subjection of mages was unlawful. That was all before the first assassination attempt.
That was before Allegra heard the demonic shrieks. All before everything changed. Now Allegra and her personal guards race to stabilize the peace before the entire known world explodes into war with not just itself, but with the abyss from beyond. So much for demons not being real.