Dark Days — Caitlin Kittredge

Jack Winter and his girlfriend Pete Caldecott have encountered a lot of strange creatures in the Black -- primordial demons, hungry ghosts, witch hunters, and the Prince of Hell himself, Belial. When Belial asks Jack for one last favor to help him keep his throne, Jack may have finally met his match because Belial's rival is something that no one -- human or demon -- has ever seen before…

There's a revolution brewing in Hell, and Jack might be the only one who can stop Belial's rival from ripping a hole between the Black and the mortal world -- a catastrophe that could be worse than Armageddon. But to win, Jack will have to do the one thing he swore he never would: become a servant to the Morrigan, and risk losing everything he knows and loves…including Pete.