In the Dark by Brea Viragh

The One Who Walks in Darkness is looking for a replacement. And she’s found one.

Karsia Cavaldi is the youngest of her line and the sweetest of the sisters. At least, she was, until evil struck a killing blow. The immortal embodiment of ancient darkness was not the sort of thing she wanted to put on her resume. Unfortunately, the magical strike to her heart left her with little choice. Her powers are out of control and evil has taken root in her heart.
Professor Morgan Gautier is more than the mortal he appears to be. Half-human, half-god, he holds dominion over the dreams of mankind. And finds himself undeniably drawn to a certain auburn-haired witch. The evil he sees on the outside is nothing compared to the goodness he recognizes when he enters her dreams. One side must win, and Karsia is torn in two.

Can the god of dreams find a way to make his own come true? Or will he lose the love of his life forever?