Entropy

Ahana, a human girl with little recollection of most of her life is thrust into leading an army of peace-loving demons, angels, monsters, and humans to stop an ongoing war and save her mentor and friend.
Brook Castelli, an angel almost as old as the gods that rule her is suddenly having to deal with the fact that her lover is now a dictator, the fact that she condemned herself to hell and that she's slowly being pulled into the horrors of war.
And of course, everyone else close to both of them being dragged into it along the way.