Mere Flesh by Catherine LaBadie

Wynnifred Treloar has everything she needs to succeed as a budding occultist on the cusp of vowing her allegiance to king and country…and marrying the most tame yet influential man in society she can find. Yet everything is not enough for her when she knows more influence than she can dream of will be withheld from her because of who she is.

Breaking one rule after another, she sacrifices portions of her body to summon a demonic entity to take over the world. There should be no harm in it: demons are only dark energy, after all, and Wynnifred is a prodigy. When the demon she summons into a circle vows she’ll die screaming in his claws and omens plague her friends and estate, Wynnifred begins to realize there are worse mistakes to make than marrying the wrong society darling.

Like beginning to fall for the demon’s allure.