The Purple Emperor (Faerie Wars Chronicles)

Henry Atherton, his faerie prince friend Pyrgus, and Pyrgus's fearless sister, Holly Blue, return in this fantastic adventure to save the Faerie Realm from the evil Hairstreak and his henchmen, Chalkhill and Brimstone. With the help of forest faeries, some silk mistresses, a sewer-dwelling creature of unknown dimensions, and additional creatures magical and otherwise, the three intrepid young friends find their way from exile back to the home they all would die to preserve and protect.