Dead Beat by Leigh Bardugo

The highly anticipated, explosive conclusion to the Ninth House trilogy from #1 New York Times bestselling author Leigh Bardugo.
Hell is empty and all the devils are here...
Alex Stern made it to hell and back more than once, but this time she left the door open behind her. Demons are creeping through the streets of New Haven and the Yale campus, feeding on the souls of the living, terrifying the dead. Alex and her allies can hunt them, but it's only a matter of time before the gates to the underworld open and they face a war they cannot win. To stop it, they'll have to reach deep into the secrets of Yale and its societies, and discover the truth hidden in Alex's own blood.
While she struggles to hold onto the dream of a future Yale once offered, Alex must contend with the machinations of a corrupt Lethe Board and a deadly vampire with his own connection to the societies. But for the first time in her life, she won't have to face the fight alone. Dawes, Turner, Mercy, and even Tripp will have a role to play, none more so than Alex's mentor Darlington, the golden boy who lived so long in her head, now back in the flesh and cursed with dangerous powers of his own. Hell itself is stalking the delegates of Lethe House, and they're all about to learn that demons don't play by human rules.