Faerie Lord: Faerie Wars IV by Herbie Brennan
Two years after leaving the Faerie Realm, Henry Atherton finds a middle-aged Pyrgus and his new wife, Nymph, in Mr Fogarty's garden. They explain that a terrible plague has taken hold of the Faerie Realm, causing faeries to age so rapidly that Pyrgus had to flee in order to save himself. Determined to help his friends, Henry returns to the Realm, hoping to find a cure and perhaps win his way back into the heart of Queen Holly Blue. But the plague isn't what it seems, and when the treacherous villains of the Faerie Realm are involved, anything is possible.