Daybreaker. Undertow, #5

In the epic conclusion to the Undertow series, 18-year-old Eila Walker is no longer the last surviving Lunaterra, and her Trial, Rillin, finally has the young woman he lost so long ago, back in his arms: Elizabeth. But Elizabeth, despite being a brilliant fighter, is not exactly a thrill-a-minute. Her abrasive personality often rubs everyone the wrong way. But despite their mutual loathing, Eila must find common ground with Elizabeth, for Elizabeth is the only one who knows how to kill Therophel once and for all. But the task of murdering the fallen angel won't be easy - he's overthrown the Wrecker empire of Mayhem and taken it for himself.
Now fortified within the towering tree walls, and protected by hundreds of Linked Mortis, Therophel clearly has the upper hand. But the fallen angel, while powerful, may've underestimated the resolve of a crew of supernatural teenagers from Cape Cod .