Erin Hunter 
Warriors: Omen of the Stars #6: The Last Hope 

The end of the stars draws near. Three must become four to battle the darkness that lasts forever. . . . After countless moons of treachery, Tigerstar’s Dark Forest apprentices are ready to lay siege upon the warrior Clans. As Jayfeather, Dovewing, and Lionblaze prepare to lead their Clanmates into battle, they await the arrival of the mysterious fourth warrior who is prophesied to help lead the Clans to glory.The darkest hour the Clans have ever faced has dawned. Hopes will be shattered and heroes will rise as the warriors fight for their very survival.