Dire Blood. The Descent Series, #5

Elise Kavanagh died while killing the mother of all demons, and her fight should be over. But nothing is ever that easy where the Godslayer is concerned. Elise has been resurrected--and she came back as a demon. Now the most powerful witch in the world--the man that Elise has loved for over ten years--has been dragged to Hell and accused of violating an ancient treaty. Elise is the only one who can save James from eternal Hellfire.
It means following him into Hell to face inhuman urges, the secrets in her past, and the horror in her future, but Elise will do anything to save James. Anything. The Godslayer's fight isn't over yet. It's only just beginning...