The Halfmen of O (O Trilogy #1)
'Nick had seen the birthmark on Susan's wrist. It had two parts. Each was shaped like a tear drop, curved like a moon. One was bright red and the other golden brown.'
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.