Feud - The Demon Chronicles, #2

Serafina is the first Guardian in centuries, a human born with powers strong enough to rival any Nephilim's. Together with her friends and the Watcher, they have overthrown the Arch Demon and taken control of the demon hierarchy. Next, they have the rest of the world to contend with, as the other Masters don't play well with others. Justin is now Master of the US demons, and is determined to force change in the demon hierarchy to live peacefully with humans.
As he struggles to fight and retain his title against other warring Masters, a familiar darkness creeps into his soul. Azazel is still alive, with revenge on his mind. The former Arch Demon is furious at being betrayed, and vows to destroy everyone who defied him. His ultimate goal: plunge the demon hierarchy into all out war, destroying the planet and all of humanity in the process.