Devil May Cry ~ Sherrilyn Kenyon

This is book twenty one of the Dark Hunter series. Sin was a Sumerian God, who's powers were stolen by Artemis, then he became a Dark Hunter, under Acheron's command. Now it appears as though Sin has simply gone crazy, and since Ash refuses to kill him, Artemis sends her handmaiden, Kat to do the job. Kat soon discovers that things are not as they appear, and there are other things that she needs to do!