Midnight by Erin Hunter

Six moons have passed since Tigerstar's death and his son Brambleclaw has grown to his full extent as a Warrior. When he receives a rather disturbing prophecy from StarClan, he is meant to meet with three other chosen cats from opposing Clans. When Firestar's daughter Squirrelpaw, as annoying and nosy as she is, decides to tag along, Brambleclaw is not only running away without his Clan's consent but with the daughter of his father's worst enemy.