A Light in the Mist (Warriors: The Broken Code, Bk. 6)

The impostor's reign of terror has brought the Clans to the brink of destruction, and now, every warrior--living and dead--must unite to defeat him before he delivers his final blow. To ensure their future and protect their past, StarClan, the Dark Forest, and all five Clans must band together to turn back the tide of darkness in a battle that will decide the fate of every warrior who has ever lived.