The Forgotten Warrior (Warriors: Omen of the Stars, #5)

With a divided StarClan driving a treacherous rift between the four warrior Clans, the spirits of the Dark Forest are gaining strength. Ivypool's role as a spy becomes more dangerous with each passing day, and Dovewing is haunted by nightmares about the mountains.
Then an outsider appears in ThunderClan's midst, spreading discord and pushing the Clans farther apart. As tensions mount and Clanmates turn against one another, the warrior cats will be forced to choose whose word they can trust--before it's too late.