Maurice Gee's O Trilogy is a masterclass in fantasy fiction

Susan had always been a bit odd and never really got on with her cousin Nick, but the mark on her wrist draws them together in a frightening adventure. They are summoned to the beautiful land of O in a last-ditch attempt to save the planet from cruel Otis Claw and his followers, the evil Halfmen, who have lost every trace of human goodness and kindness.